There is no substitute for a qualified proper cable tester to run down internet connection problems.
However, Comcast has a help site with many pointers which also apply to our local Willow Valley Senior Internet system if you are tech oriented.
Skip down to check your modem signal levels. Of course, it will take a qualified technician to deal with any issues.
Don't connect your computer direct to your modem and unless you want to welcome malware into your system.