Using WiFi 6 instead of the Gateway

WiFi 6 only works if your connected devices are WiFi 6 capable as well. Otherwise, they will connect to whichever band is available and compatible. There are currently very few devices with the new WiFi 6 modules in them.

With the AT&T gateways, you should be able to put the router portion in pass through mode (similar but not the same as bridge mode) and connect a different router. If coverage is your problem, then WiFi 6 won't work for the reason I gave above and you might want to consider a Mesh WiFi router. Those work extremely well for most situations.