How can I force my phone to connect to US-based cell towers only?
It is currently impossible to do force one's phone to connect to US-based cell towers only. E.g. from Dicknose22 on https://www.reddit.com/r/AndroidQuestions/comments/j5ntyg/how_can_i_force_my_phone_to_connect_to_usbased/g7tefwh/:

Not to the best of my knowledge, even with total root access, your phone and the tower communicate on what to connect to you do not have power to override what your carrier decides you should connect to.