Having trouble setting up newer third-party modem

I've been having a rough time trying to switch out my current third-party modem with a newer third-party one. I've tried both the phone number, the Xfinity assistant, and the app. The phone number tells me to use the assistant. The assistant won't let me connect to an agent - no matter how many times I ask for someone. And the app will not let me connect to the account - and the app still shows the older address where I used to have internet, but won't let me update to the current address where I live now. Could someone help me please?