Cannot change IP address in TCP/IP game

Description

Playing direct connection via TCP/IP, behind a router. We finished for the night and quit. When restarting the game another day as host, the IP identified was my LAN IP, not WAN. The Change IP button itself is greyed out, and unselectable. On the player side, Host IP was changeable.

Unplugged from my router, directly to PC. Had DHCP problems, rebooted modem, which grabbed an IP different from the one we originally started with. Even after disabling firewall, players could not connect to me.

Is there more than just the two ports that need to be forwarded?
Can the Change IP on the Host side always be active?