With a managed switch, you can put the modem and the LAN on separate VLANs then trunk them into the Pi's single port. The Pi would take in wan on one network and route it out to lan on another network using the same cable. This can be expanded to include guest networks etc. by adding more VLANs. The overall concept is called router-on-a-stick or one-armed router.
If your switch is unmanaged you'll need two ports on the Pi and also some way to power up the modem while it is on the separate network.