None of what you've suggested is applicable. I do not have a "combo" unit from my ISP. There is only two wooden/drywall walls between the main and point. The connection between the main and point doesn't seem to get affected. There are no "special characters", there is no IPv6, the DNS has always been the default (which is Google).
After resetting, the router will work OK for a day or so, then it will begin refusing more connections. I'll get a notice from Google Fi on my phone that my data usage has spiked, I look and see I'm not connected to WiFi. I try to connect to WiFi and the router refuses the connection. It isn't the phone (Pixel 6 Pro), I've rebooted it, still no connection, router refuses. Same with husband's Pixel 6, same with all the other devices (Tesla Powerwall, smart oven, Ring cameras, etc.), they're all "frozen". I can't access the router's settings because the Home app just sits there with the blue circle. I have to physically unplug the main router, plug it back, and wait for it to reboot. Then, everything connects and works normal again for a day or so.
The main router does get very warm to the touch, almost hot. It is not in a hot room, the temp is 68 degrees F, well ventilated.
And none of this addresses the second problem, which is a design choice by Google, that makes this router system impractical. If on the rare occasion my ISP goes down, Google has chosen to disable the local network as well. That's absurd, but that's what they do. No other router system I've ever worked with does that.
