Terms of service, probably. Never bothered to read it, I'm sure I break their terms of service 6 ways to Sunday. At the same time I've been doing this type of bypass with AT&T for years and never heard of anyone getting blocked for it either. I'm sure someone somewhere has though but I suspect they are much more interested in those abusing their connection or causing legal troubles than someone paying but silently using a different modem.

As for the one I'm currently using, https://pon.wiki/guides/masquerade-as-the-att-inc-bgw320-500...

There is a Discord "8311" which has a 10G-PON US channel for a community around this.