David needs to extend the WiFi in his apartment building. Will an extender do the job? Leo says it will, but at the cost of cutting bandwidth speed in half, because the base station is relaying the signal and spending half the time talking to the router. Leo recommends going with a MESH router. Eero is a good one, as is NetGear Orbi. But there are plenty more. Just about every router company makes a mesh model now.
David will also have to use AT&T's own router for UVerse. He'll want to turn off DHCP and just let it route the traffic to the Mesh router. If he can't, he can set the mesh router into bridge mode. Here's a link to how to use an Eero Mesh router with UVerse - https://support.eero.com/hc/en-us/articles/207988076-Setting-up-eero-with-AT-T-U-verse