You need your local IP, you get that in the router(should be a devices list) or by opening a command prompt and typing ipconfig (local ip listed under ipv4 address in list). The whatsmyip site will give you your external IP which identifies your modem externally(will be the same from any PC on your network). Your local IP is what the modem/router uses to determin what goes where inside your local network. Example: my internal IPs are 10.0.0.2, 10.0.0.3, ect, while my external is 98.214.165.**.
Use the local ip(from ipconfig) that you get from the pc you're trying to connect to, in the starmade game on the pc you're trying to connect with.
You might even be able to get away with using localhost: (port number), but idk if starmade will work with that, some programs/games do.