Connecting mac directly to modem - self assigned IP

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.