We don't know much about the OP's WiFi setup other than he chose to use his own equipment and not to use the carrier's gateway - which would broadcast its own, stronger, passpoint SSID in his apartment and likely these issues would go away.
Consider the following scenario: the OP's router is flaky and disassociates stations during which time the phone gets kicked off then "sees" the carrier's SSID from the neighbor and joins it. Now it's on that network. Suddenly this becomes a self-created problem. The solution is to not use Xfinity mobile or any other "Wi-Fi first" MVNO.
They're complaining that it's not using their own Wi-Fi and causing real issues for their family members.
Trying to justify this behavior because "well, the ToS maybe said something about this" rightly deserves to get downvoted, as do your other points.
None of what you call out changes the fact that:
1. The OP is experiencing real issues, preventing them from using their own Wi-Fi at home
2. This is because of a configuration setting they cannot change on a device they own.
What SIM they're using, what ToS they've signed, none of that should matter when OPs goal is just "I want to be able to use my own Wi-Fi at home".