WiFi keeps disconnecting on all devices for a few seconds before reconnecting

I'm having an issue where all of the devices on my WiFi simultaneously disconnect and then reconnect after a few seconds.

I have AT&T Fiber with 1Gbps speeds, and I recently had AT&T come out to replace the modem/router combo because I assumed that was the issue, but that didn't fix the problem.

The devices that are connected to the router via ethernet don't seem to lose connection. The AT&T tech wasn't sure what would be causing that issue, and assumed replacing the modem/router would fix it.

I have a Windows desktop and Samsung TV connected via ethernet, and then a Google TV, Google Pixel 5, Google Pixel 4a, Macbook Pro, Alienware laptop, Dell laptop, Lenovo Thinkpad that are all connected to the WiFi (but normally on at the same time). I have this issue with just the Google TV and a single phone on the WiFi, so I don't think it's a congestion issue.

Any ideas on what could be causing this? Or what I can do to fix the issue?