How to activate with own equipment

I cannot activate service with my own modem which is on the list of approved modems.   My modem is setup and I've gone though all the steps to activate but get an error message.   I have not cancelled my old ISP yet because I cannot have a gap in service since I am working from home full time.   Does this need to be done first?  I have tried for over two hours to get a live person but I can only get through to an automated message that tells me I need to power cycle my modem and someone will call me back within 10 minutes.  That was over 12 hours ago and no one has called me back.  I have called at least 5 times and get the same message.  At this point I cannot risk cancelling my old service to run into this issue and go without service.  I cannot even setup installation by calling.  Can you help me?  I'm really having second thoughts about continuing service with the level of customer service Comcast is providing.