@DavmoNet There's a newer FW version for your VR1210v - here. Upgrade and test if any difference. You can revert back to your current version if needed. Here's how to upgrade the FW manually.

@Anan_265 I see that your ISP is using PPPoE connection. In this case you should contact them and ask if accessing their ONT modem is allowed/possible. You also have a double NAT introduced by this...

@GiovanniG11 Use the app to configure the Deco. Just follow the steps in the guide I posted to switch the Deco in AP mode.

@StefanoL There's a newer FW version - use this link for downloading - you need the EU version. Then test if the USB modem is recognized. If not, then you can't use this USB modem.

@Anan_265 Unless your ONT Modem is in bridge mode, Archer C20's WAN default gateway should be the ONT Modem LAN IP address:

@GiovanniG11 Having in mind your topology, you can set Deco M9+ in AP mode - this should solve your issue - here's the configuration gide.