Switching from wan to USB tethering or hotspot


first some explanation about my config.

Modem ISP ↔ AR750S-ext (3.104 with adguardhome) ↔ LAN

AR750 is static ip
is configured in DMZ
is openvpn PIA for some clients.
is Adguardhome with cloudfare DoH

I have tested USB tethering/hotspot on a testbench and it worked great. That is, without a WAN cable attached.
Today i had ISP troubles. Modem would not connect to internet. I connected hotspot to ar750 but no internet on router.
Then USB tethering, and no internet.
Disconnected wan cable and changed port to lan, same, no internet.

That’s why I bought the ar750s initially to switch fast from WAN to USB tethering or mobile hotspot.

Errors like :
Error: control/version.json | Couldn’t get version check json from https://static.adguard.com/adguardhome/release/version.json: *url.Error Get https://static.adguard.com/adguardhome/release/version.json: synthetic.wrap: all upstreams failed to exchange request, cause: read udp> i/o timeout (hidden: read udp> i/o timeout, dial udp [2620:fe::10]:53: connect: permission denied, dial udp [2620:fe::fe:10]:53: connect: permission denied) | 502

Can somebody help me how to switch fast from wan to usb tethering/mobile hotspot?

Thank you very much,

grtzz tom

Cable Modem <-> AR750S-Ext (3.104 standard) WAN In, LAN Out <-> 8 Port Switch <-> Devices

The few times we have had internet fail have swapped from cable to USB tethering quickly as follows:
Disconnect WAN from Cable Modem, Connect Phone via USB cable to AR750S-Ext USB
Set phone to USB tethering, Login to AR750S-Ext and Enable Tethering, Internet back & running