I'm assuming that these are really not an OPNSense questions. Have you looked into T Mobile internet for either yourself or parents. This would definitely separate the activity. Short of that, buy a better router and put the ATT equipment in bridge mode to allow your router to control the network. One of the primary reasons I installed software like OPNSense was the ability to create separate networks (subnets). I started with Untangle.
Since this is temporary, my first option would be T Mobile. Good luck.
Since this is temporary, my first option would be T Mobile. Good luck.