Wifi is not stable. Works fine one minute, but disconnects or very slow the next (wired is fine)

About 10 days ago, my wifi became very unstable.  Working fine, then drops or does not connect at all.  Same behavior across all of our devices (iphones, ipad, laptops, xboxes, etc)  I could hardwire in and speeds were up in the 600 - 850 mbps up and down.  I called ATT, they did standard, restart everything and let me "send a signal" to your system.  If I rebooted, or they forced a reboot, then the wifi would work for maybe an hour or so, but back to same behavior.  After a reboot, it would go through about 30+ minutes of complaining my wifi password was not correct for each device...but that would eventually clear up.  ATT shipped me a new router.  It behaved the exact same way.  I called back and they sent out a tech.  The tech replaced the new modem with a "newer" modem.  He was able to get the system working for the 1.5 hour he was here, but every speedtest we ran would fluctuate over 200 mbps between runs when doing on wifi.  Eventually, he called back to his supervisor, who pulled the "we can only guarantee hardwired ethernet when using our special only testing on ATT's direct network..."  Anyhow, my goal was to get a consistent wifi connection, even if it was only 50% of the speed I am paying for...  He left and the next day, the behavior is exactly the same, I pretty much can only hardwire, as my other devices will get up to 400 mbps and then immediately be at 5 mbps (or not connect at all).

What I have tried:

- Many Reboots (router and the main connection box at the wall)

- 3 different ATT routers at this point

- reset to factory defaults

- ran apple wifi diagnostics and manually set the wifi channel to the best recommended channel

- moved the router to a new location after removing everything that was near the router location with no luck  (although I would literally be standing 2 feet from the router when testing)

I am unaware of any new devices that may cause interference, but it is odd that even isolated and with the device practically on top of the router, I still have the same issue.  (signal strength is alway strong, but wifi internet connection is horrible or not working at all).

Ideas?