What’s your use case? I don’t think the WiFi from the Router will be too bad for like, Lemmy on mobile, even in Bedroom 2. For videos maybe. If you want to get Internet on TVs and desktop PCs, just use the Ethernet cable directly, why bother with WiFi.

You may want to try-before-you-buy, by placing an ordinary WiFi router in different locations, and using a cellphone app like “WiFi Analyzer” to see what kind of signal levels you get. (You don’t need ISP connectivity, the router will still broadcast without it. . .)

Depending on the dimensions of your rooms, an access point in the hallway by the door to bathroom 2 might be able to cover both bedrooms on its own, leaving the AT & T router (if it has WiFi) to cover the living room/dining room.