AFAIK, in America, ISPs are required to permit user-provided modems as long as those modems are technologically compatible. I believe the Television Viewer Protection Act of 2019 did this, although it was already the norm for ISPs to permit third party modems before this.. I guess because they knew if they pushed the issue they'd lose anyway, given the precedent of telephones, cable cards, etc.
The problem then with Starlink is nobody is manufacturing compatible third party Starlink terminals, at least yet.
I'm not sure if that applies to Fiber connections. I had my own fun with a firmware update causing latency issues with ATT Fiber's gateways that was fixed several months later [1]. I was using the combo ONT + Router originally and had to switch to dedicated ONT.
I ended up extracting 802.1x certificates with this[2] or a similar tool and interfacing directly with the ONT using OPNSense [3]. I was so angry I filed an FTC complaint because I had to do this bypass to do my job (the latency was so bad).
The problem then with Starlink is nobody is manufacturing compatible third party Starlink terminals, at least yet.