C6250 bad connection

We recently decided to stop renting our modem/router from Comcast. The one from Comcast always worked well and never had a problem. Never had to unplug and plug it back in. Always got high mbps numbers when I did tests on my phone. We pay for 200mbps. Every since we got the c6250 we always have problems. More often than not we have bad connectivity. Just now I tested it at 12 Mbps. I can't even download new app updates on my iPhone 7. When I unplug it and plug back in. Nothing changes. I know the internet is fine because my Xbox one is plugged directly into the c6250 and works great. I don't know much about this stuff so I need help. This is really annoying because I'm paying for the much more expensive internet and not getting the speeds I need. Any help would be very appreciated!