1. netgear (and other consumer networking companies) is overwhelmed right now due to a vast increase of people working from home as well as their own call centers health/status. it is going to be tough to get a hold of support
2. What exact issues are you having? describe whats going on in details so we can better help you. Stating "it drops" doesn't help. Does wireless drop, is it connected but saying "no internet available", etc. the more you tell us, the more we can help
3. comcast has been using docsis 3.1 devices. There a bit more tolerant of sketch connections. The c6250 you bought is a docsis 3.0 device. Its more sensitive to a bad line. If you have splitters, amplifiers, attenuators in-line for your coax, remove them. Also try to move the router to right where the coax enters the home. This prevents wiring within the home from being the issue.
4. If you log into the C6250, click on the internet icon, it should bring you to the modem's connection page back to the isp. Take a screen snip of this. It shows us a snapshot of your connection back to the isp. This is kind of key at helping determine if your connection is decent.