It sounds like your problems are caused by a faulty connection. Unfortunately, it seems that Comcast technicians do not fix faulty connections, so it is up to you. If that faulty connection is under ground, 1) it will be very difficult to find, and 2) it will be very difficult to fix.
"Can I cancel"
I don't think that's allowed.
I signed up for xfinity internet two weeks ago. it has been really unstable. I bought a new SB8200 and it still doesn’t help. It could down 3 times a day. I check status in 2 days it accumulate more than 20K, K as thousand, uncorrectables. Even log had ton of T3 time out stuff. It is essential in this COVID era. Can I cancel the service? ATT fiber was stable for me. With such bad service, Comcast is breaking the contract.
What do the modem's signal stats look like ?
Please post the *Downstream Power Level*, the *Upstream Power Level*, and the *SNR* (Signal to Noise Ratio) numbers.