In that case there should be no special configuration needed. Did you make sure the modem's configured on its own subnet?
Example setup with multiple modems with accessible web interfaces:
WAN_CABLE is connected to an Arris cable modem on em3. The router uses DHCP and gets given a public IP address on that port. The modem's web interface is available on 192.168.100.1. As this is on another subnet, when I access that it just works(TM).
MODEM_DSL is connected to a Vigor 130 modem on port em1. The router uses DHCP and gets given a private IP address on that port. The modem's private IP address is set up at 192.168.101.1. Accessing 192.168.101.1 also works.
WAN_DSL is a PPPoE connection over port em1. The modem web interface isn't accessible over that.