Unclear what "everything else works fine" means here. The CM1100 is a pure cable modem, no NAT router, no WiFi - as such you can connect only one device direct to it, typically a computer as required for the initial config to your cable TV provider. For anything more, you need a WiFi router.
Most cable ISP assign only one public IP to such a device. Even if there are multiple LAN port, only one device can be connected direct. The reason for two LAN ports is the ability to run port aggregation to a capable NAT router allowing to reach total bandwidth beyond of what a single Gigabit Ethernet link allows.
