Identify the IP address needed to log into the modem, that is the one that the modem holds for http / TCP connections outside the public IP and pppoE protocol. For cable modems it is usually 192.168.100.1, for DSL it can be others typically 192.168.0.1
If this IP overlaps and conflicts with your LAN you will need to change one of them.
Once that is sorted, create a dummy interface to reach the modem as described in the "WAN via PPPoE" section of the guide that @trendy linked. It needs a static IP and netmask that covers the modem's adminstration network but is not the same (e.g. 192.168.0.2, netmask 255.255.255.0). As the guide describes, place this network in the wan firewall zone because you don't want to trust the modem, you will only make outgoing connections to it.