Internet keeps dropping or slowing down over and over each day

For weeks and weeks now, we've had trouble with the internet dropping or slowing down considerably multiple times each day. I work from home, and my teams calls are constantly interrupted. I have to put them on my phone signal, not my home internet, to make it manageable.
Use of collaborative web tools can be slow and lag intermittently. Other times it's fine.
In addition, my family is having trouble with online gaming both in the game itself and in their communication apps. 

A tech came out to our house and made some improvements to the signal we were getting from the source to our modem, and while that seemed to help, it did not fully solve the problem. We have since replaced the modem and the router, but the problem still remains. I can't seem to get another tech appointment set up via the chat service on the xfinity site, so I'm here asking for help instead. We don't know what else to try. I've worked from home for years now and never had this issue until the past month or so.

And we have already tried all the troubleshooting lists posted. Still no help.