You also mentioned it might be the wifi on the modem. I'll see if it will let me put a AP on the modems ethernet port. But, this thing is so crippled. I don't know if it will work.
I can't read... yeah its on of the all-in-one home internet solution.
I recall there is some "passthrough" mode. So you can plug in your router. Or use it without passthrough mode with an AP. But it for sure be good to some test with laptop connected to one of the ethernet ports on the BGW530, as that remove Wi-Fi being involved in what's going with AT&T's network for internet.
Also, I think most settings are control the AT&T app, which are limited... But if there are wi-fi channel selectors, that may help so you can avoid a DFS channel/auto and/or limit the channel size to 20 or 40Mhz for Wi-Fi.