Trying to diagnose a speed issue. I thought that it could be old lines not capable of 300mbs, but the Xfinity rep said that all lines were capable of 1GB, everywhere. So, I can connect to the internet with my mac connected to my wifi router via ethernet, and the router connected to the modem. When I plug my mac directly to the modem via ethernet to thunderbolt adapter, I can't connect. My mac tells me that my IP address is self assigned. I'm not sure how to fix this. I tried to go into settings and tell it to renew the DHCP lease, but that didn't change anything.
Try power cycling the modem.