Welcome to the community! Sorry to hear about your troubles.
Is your AT&T gateway still in router mode? If so, do you know if it’s still on the default subnet range (192.168.1.254/24)? That would be my first guess as the shipping firmware is known to have issues when it shares the same subnet with the upstream network.
You can try the automatic firmware recovery, and after several attempts it may be successful and upgrade to a newer firmware, or you can change the subnet on your AT&T gateway (so change from say 192.168.1.254/24 to 192.168.2.254/24, etc). Changing the subnet would be the easier route in this case.
To perform automatic firmware recovery, disconnect power from Route10, press and hold reset, reconnect power while still holding reset, release reset when the top LED comes on. This should take several minutes. It may or may not work the first time, which can be frustrating,
Also, if the gateway is still in router mode… you’ll be in what’s called double NAT, which is not recommended. It’s really ideal to put the modem in bridge mode if possible. Anyway, first things first is to get it online and updated.