5GHz WiFi connection keeps vanishing. AT&T Router.5268

I have an AT&T 5268AC router. My 2.4GHz connection works fine as does my PC over Ethernet. But for some reason, my 5GHz connection keeps appearing/vanishing from my list of available WiFi connections.

Any device I try to connect via WiFi, I must connect at 2.4GHz. When viewing the list of available connections, the 5GHz connection appears for a second, disappears for a few seconds, rinse, repeat. It never stays active long enough to connect.

This started happening several weeks ago. Before that, I used my 5GHz connection exclusively for everything.