My config is as follows:

config interface ‘modem’
option proto ‘wwan’
option country ‘country73’
option sp ‘isp0’
option device ‘/dev/ttyUSB3’
option service ‘umts’
option dialnum ‘*99#’
option apn ‘broadband’
option username ‘WAP@CINGULAR.COM’
option password ‘CINGULAR1’
option metric ‘40’

but I too am not connecting. I also tried this:

config interface ‘modem’
option proto ‘qmi’
option country ‘country73’
option sp ‘isp0’
option modes ‘lte’
option dialnum ‘*99#’
option apn ‘broadband’
option username ‘WAP@CINGULAR.COM’
option password ‘CINGULAR1’
option metric ‘40’
option device ‘/dev/cdc-wdm0’
Why are you having the above in the section for ‘wan’, should this not be ‘modem’?

For others, some additional reference

https://wiki.openwrt.org/doc/uci/network#protocol_qmi_usb_modems_using_qmi_protocol

https://wiki.openwrt.org/doc/uci/network#protocol_wwan_usb_modems_autodetecting_above_protocols

I also notice that for proto wwan there is option ‘service’, but for proto qmi there is option ‘modes’, which you have embraced.

With the default GUI config the logs specifically ref the Quectel EC20:

Wed Oct 26 14:42:08 2016 daemon.notice netifd: Interface ‘modem’ is setting up now
Wed Oct 26 14:42:08 2016 daemon.notice netifd: modem (2759): wwan[2759] Using proto:wwan device:/dev/cdc-wdm0 iface:wwan0 desc:Quectel EC20
Wed Oct 26 14:42:08 2016 daemon.notice netifd: modem (2759): Waiting for network registration
Wed Oct 26 14:42:08 2016 daemon.notice netifd: modem (2759): Starting network broadband
Wed Oct 26 14:42:09 2016 daemon.notice netifd: modem (2759): Unable to connect IPv4
Wed Oct 26 14:42:09 2016 daemon.notice netifd: modem (2759): Unable to connect IPv6
Wed Oct 26 14:42:09 2016 daemon.notice netifd: modem (2759): Unable to connect
Wed Oct 26 14:42:09 2016 daemon.notice netifd: modem (2858): Stopping network
Wed Oct 26 14:42:10 2016 daemon.notice netifd: modem (2858): Command failed: Permission denied
Wed Oct 26 14:42:10 2016 daemon.notice netifd: Interface ‘modem’ is now down

With the QMI config I get this:

Wed Oct 26 14:42:43 2016 daemon.notice netifd: Interface ‘modem’ is setting up now
Wed Oct 26 14:42:43 2016 daemon.notice netifd: modem (3858): Waiting for network registration
Wed Oct 26 14:42:49 2016 daemon.notice netifd: modem (3858): Starting network broadband
Wed Oct 26 14:42:49 2016 daemon.notice netifd: modem (3858): Unable to connect IPv4
Wed Oct 26 14:42:49 2016 daemon.notice netifd: modem (3858): Unable to connect IPv6
Wed Oct 26 14:42:49 2016 daemon.notice netifd: modem (3858): Unable to connect
Wed Oct 26 14:42:49 2016 daemon.notice netifd: modem (3951): Stopping network
Wed Oct 26 14:42:49 2016 daemon.notice netifd: modem (3951): Command failed: Permission denied
Wed Oct 26 14:42:49 2016 daemon.notice netifd: Interface ‘modem’ is now down