Constantly Dropping Signal Several Times Each Day

After my old modem was no longer supported by Xfinity, I went to their "approved" list of modems on their website and bought the Motorola MB7621 modem (my old modem was also a Motorola and I never had any trouble with it at all) from their list.  I also bought a new router, the TP-Link AC 1750.  Initially, they seemed to work just fine, but within days, I kept getting "Cannot connect to the server" messages.  I called Xfinity and a service tech came out and said it was my old cable lines buried underground... that they don't even use those any longer and that moisture and cracks had most likely gotten into the cables.  He assured me that once they ran new lines, my problem would be resolved.  After running new lines, the problem was not resolved.  Another call and another tech came out and checked everything out.  I even purchased a second router to see if that was the issue, but it was not.  When he was done, he ran some diagnostics on his phone and everything seemed to be working well.  By the way, when it does work, it works great, but the constant drops when trying to work from home do not allow me to do my job and are very frustrating when I'm paying lots of money for Xfinity service that I cannot even use.  Shortly after the second tech leaves, I get the same "Cannot connect to the server" message and lose my signal.  After a third call to Xfinity, they send out a "senior" service tech who says the problem is my outdated wall outlet connection.  He replaces the wall outlet and cables to my router and modem, checks the outside connections, runs some diagnostics on his phone, and says everything looks great.  Again, a few hours later and I lose my Internet connection.  I decided to purchase another modem from Xfinity's "approved" list, the Netgear CM1000, hoping it might be the problem.  At first, it seemed to work well, but hours later, I lose my WiFi connection even though the signal indicates I have full strength.  I constantly have to reboot my router, so I try changing the channel on my router to see if that helps.  Not really... and now my MODEM loses connection constantly, making me reset it several times each day, and sometimes within minutes of just having reset it.  I am certain that the problem is NOT my new modem or router... that it IS an Xfinity issue, but I am exhausted taking my time to chat and call them to ATTEMPT to fix the problem!  I've literally spent hours on the phone and had three different techs come out to my house and still have these major issues that do not allow me to get my work done from home or my son to take his online college class this summer.  Honestly, it makes me wonder if ANY of the third-party purchased modems or routers actually work with Xfinity service in their attempt to get customers to HAVE to RENT Xfinity's modem/router combos!  I have read so many reviews on practically every modem listed on Xfinity's "approved" list and ALL of them have problems with connectivity with Xfinity service... ALL of them!  Customers just like me who are highly frustrated after trying multiple "approved" modems and routers, having Xfinity service techs come out and tell them everything looks fine and it must be their modem or router, hours on the telephone talking with Xfinity reps who are absolutely useless, etc. -- and still we all continue to have these connectivity issues.  Unfortunately, I think I am done with Xfinity unless someone very soon can get this issue resolved WITHOUT me having to RENT one of their devices to get the connection I need in order to work from home AND that I am paying good money for!  PLEASE HELP!!!!!!