Unable to activate or change internet plan

I recently moved and have my own modem, but the Xfinity app won't activate it no matter what I do including check/tighten coax cable, restarting it, etc. I get internet so obviously it's working, but it just won't activate. I've talked both to an live chat rep and called to talk to a person over the phone but still no activation. Because of this, I can't change my plan or update it to a newer one and haven't been able to since I put in the move order last month. When I go to change my plan, I get through to the add-ons then get an error telling me to try again.
My bill is coming up and I would like to change my internet plan, is there any way I can get my modem activated? When I talk to the chat reps as well, they're giving me completely different deals (not what I asked for or trying to get me to bundle mobile despite me saying multiple times that I do NOT want to) than what is being offered online to me.