1. You had a modem that wasn't on their approved device list. It needs to be on this list to receive firmware customized to their network parameters. https://www.xfinity.com/support/devices/
2. (the most likely scenario) You were switching the macaddr of your router's WAN port every time you connected. For IPV4 Comcast was always set up to only allow one macaddr to bind to a public address at a time, you needed to trigger a dhcpcd release on the old macaddr before reconnecting or clone the mac of the old device if switching.
1. You had a modem that wasn't on their approved device list. It needs to be on this list to receive firmware customized to their network parameters. https://www.xfinity.com/support/devices/
2. (the most likely scenario) You were switching the macaddr of your router's WAN port every time you connected. For IPV4 Comcast was always set up to only allow one macaddr to bind to a public address at a time, you needed to trigger a dhcpcd release on the old macaddr before reconnecting or clone the mac of the old device if switching.