Have you read this topic?
T-mobile seems to be the recommended network in many areas, but implementations may vary from region to region.
To update your OS I think the solution would be to remove the SIM temporarily and connect to a stable Wi-Fi network. This isn’t recommended as a rule, because normally carriers and manufacturers work together to iron out any carrier-specific bugs and may even have to distribute variants for some carriers. So, turn off the phone, remove the SIM, and turn on the phone again. Then check for OS updates.
As for Wi-Fi 6 I don’t think that the FP4 is compatible (specs indicate: Wi-Fi 802.11 a/b/g/n/ac). Make sure that your router is offering Wi-Fi 5.