I have the exact same problem with 4.4.10 (modem firmware RM520NGLAAR03A03M4G_01.201.01.201), after 1-2 days I completely lose connection and if I try to manually reconnect it connects and then suddenly disconnects (only a reboot fix the problem):
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:027] QConnectManager_Linux_V1.6.3.1
Tue Aug 13 07:35:49 2024 kern.err kernel: [189345.570657] [I][mhi_netdev_open] Opened net dev interface
Tue Aug 13 07:35:49 2024 daemon.notice netifd: Interface 'modem_0001' is now up
Tue Aug 13 07:35:49 2024 daemon.notice netifd: Interface 'modem_0001_4' is enabled
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:028] Find /sys/bus/usb/devices/1-1.2 idVendor=0x2c7c idProduct=0x801, bus=0x001, dev=0x004
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:028] network interface '' or qmidev '' is not exist
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:028] netcard driver = pcie_mhi, driver version = V1.3.4
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:029] qmap_mode = 1, qmap_version = 9, qmap_size = 15360, muxid = 0x81, qmap_netcard = rmnet_mhi0
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:029] Modem works in QMI mode
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:037] cdc_wdm_fd = 7
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:051] Get clientWDS = 15
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:056] Get clientWDS = 16
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:059] Get clientDMS = 1
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:062] Get clientNAS = 3
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:064] Get clientUIM = 2
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:068] Get clientWDA = 1
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:071] requestBaseBandVersion RM520NGLAAR03A03M4G
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:075] qmap_settings.rx_urb_size = 15360
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:075] qmap_settings.ul_data_aggregation_max_datagrams = 11
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:075] qmap_settings.ul_data_aggregation_max_size = 8192
Tue Aug 13 07:35:49 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:075] qmap_settings.dl_minimum_padding = 0
Tue Aug 13 07:35:49 2024 user.notice mwan3[16636]: Execute ifup event on interface modem_0001 (rmnet_mhi0)
Tue Aug 13 07:35:49 2024 user.notice mwan3[16636]: Starting tracker on interface modem_0001 (rmnet_mhi0)
Tue Aug 13 07:35:49 2024 user.notice mwan3track[6305]: Stopping mwan3track for interface "modem_0001"
Tue Aug 13 07:35:51 2024 user.info mwan3rtmon[5884]: Detect rtchange event.
Tue Aug 13 07:35:52 2024 kern.info kernel: [189348.701657] net rmnet_mhi0: link_state 0x0 -> 0x1
Tue Aug 13 07:35:52 2024 kern.info kernel: [189348.706607] IPv6: ADDRCONF(NETDEV_CHANGE): rmnet_mhi0: link becomes ready
Tue Aug 13 07:35:52 2024 daemon.notice netifd: Network device 'rmnet_mhi0' link is up
Tue Aug 13 07:35:52 2024 daemon.notice netifd: Network alias 'rmnet_mhi0' link is up
Tue Aug 13 07:35:52 2024 daemon.notice netifd: Interface 'modem_0001_4' has link connectivity
Tue Aug 13 07:35:52 2024 daemon.notice netifd: Interface 'modem_0001_4' is setting up now
Tue Aug 13 07:35:52 2024 daemon.notice netifd: modem_0001_4 (17235): udhcpc: started, v1.33.2
Tue Aug 13 07:35:52 2024 daemon.notice netifd: modem_0001_4 (17235): udhcpc: sending discover
Tue Aug 13 07:35:52 2024 daemon.notice netifd: modem_0001_4 (17235): udhcpc: sending select for 10.51.51.126
Tue Aug 13 07:35:52 2024 daemon.notice netifd: modem_0001_4 (17235): udhcpc: lease of 10.51.51.126 obtained, lease time 7200
Tue Aug 13 07:35:52 2024 user.notice firewall: Reloading firewall due to ifup of modem_0001 (rmnet_mhi0)
Tue Aug 13 07:35:52 2024 daemon.notice netifd: Interface 'modem_0001_4' is now up
Tue Aug 13 07:35:52 2024 user.notice mwan3[17358]: Execute ifup event on interface modem_0001_4 (rmnet_mhi0)
Tue Aug 13 07:35:52 2024 user.notice mwan3[17358]: Starting tracker on interface modem_0001_4 (rmnet_mhi0)
Tue Aug 13 07:35:55 2024 user.info mwan3rtmon[5884]: Detect rtchange event.
Tue Aug 13 07:35:55 2024 user.notice firewall: Reloading firewall due to ifup of modem_0001_4 (rmnet_mhi0)
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:092] requestGetSIMStatus SIMStatus: SIM_READY
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:095] requestGetProfile[1] casa.postemobile.it///0/IPV4V6
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:098] requestRegistrationState2 MCC: 222, MNC: 10, PS: Attached, DataCap: LTE
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:101] requestQueryDataCall IPv4ConnectionStatus: DISCONNECTED
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:103] requestQueryDataCall IPv6ConnectionStatus: DISCONNECTED
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:113] requestSetupDataCall WdsConnectionIPv4Handle: 0xe22e85e0
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:120] requestSetupDataCall QMUXResult = 0x1, QMUXError = 0xe
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:120] call_end_reason is 1
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:120] call_end_reason_type is 2
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:120] call_end_reason_verbose is 210
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:49:120] try to requestSetupDataCall 5 second later
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:50:132] requestSetupDataCall QMUXResult = 0x1, QMUXError = 0xe
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:50:132] call_end_reason is 1
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:50:132] call_end_reason_type is 2
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:50:132] call_end_reason_verbose is 210
Tue Aug 13 07:36:00 2024 daemon.notice netifd: modem_0001 (16630): [08-13_07:35:50:132] try to requestSetupDataCall 10 second later
Tue Aug 13 07:36:00 2024 cron.err crond[18673]: USER root pid 18249 cmd . /lib/functions/modem.sh;check_ip
Tue Aug 13 07:36:00 2024 cron.err crond[18673]: USER root pid 18250 cmd sleep 30;. /lib/functions/modem.sh;check_ip
Tue Aug 13 07:36:00 2024 cron.err crond[18673]: USER root pid 18251 cmd . /lib/functions/modem.sh;modem_net_monitor
Tue Aug 13 07:36:26 2024 user.notice mwan3[19502]: Execute ifdown event on interface modem_0001 (rmnet_mhi0)
Tue Aug 13 07:36:26 2024 user.notice firewall: Reloading firewall due to ifdown of modem_0001 (rmnet_mhi0)
Tue Aug 13 07:36:27 2024 user.info mwan3track[16979]: Detect ifdown event on interface modem_0001 (rmnet_mhi0)
Tue Aug 13 07:36:32 2024 user.notice mwan3track[16979]: Interface modem_0001 (rmnet_mhi0) is offline
I came directly from version 4.4.8 wich had absolutely no problems, now I am trying 4.4.9.