Xfinity Modem is Junk Dropping signal every couple minutes

I became a customer in January.  My brand new super fast modem worked great for about 30 days.  Then it started dropping off line every couple of minutes.  It made watching television and playing video games so frustrating.  I had a very hard time getting through to an actual person but they always said the same thing.  Restart the router. Everything is fine.  The problem is in the line.  After months no one could fix the problem.  I was moving so I requested a brand new router.  It was hooked up at my new address.  It worked perfect for 28 days.  Then it started.  My son almost timed out of his college exam.  Once again I started the resetting of the modem.  All lights are always white.  It does not display that it is off line.  Once again I was told it must be a cable issue.  Need to have someone check outside the home.  Now after 2 weeks it is bad like the last modem.  Can't even watch TV again.  I told customer service that a want another brand new modem so that I get the service I'm paying for.  This representative refused.  Said it is not a permanent fix.  However, due to COVID can't send someone to fix it either.  Next college exam is in 10 days.  I'm going to have to use my cell pone as a hotspot for fear of failing a test from lack of Xfinity signal. This cannot be a coincidence.  These new modems only work for a month and then render all the devices useless.  We all use our cellular while at home.  This is ridiculous. How does this problem get fixed?