As Bill says, there may be a guest network. To join in with the game, you must be on the same network as the other PCs in the game. Are all the PCs connecting to the router the same way? If they are are connected by Ethernet then they should all be on the same subnet starting with 192.168.0... However, if they are all connected by WiFi, then you should check that they are all connected to the same WiFi network. If your PC is the only one connected by WiFi, see if you can use an Ethernet connection, as the WiFi you are using may be a guest network on a different subnet starting 192.168.1...
Opening ports when you are on a private network with your friends simply means configuring your PC's firewall so that it does not block incoming connections on port 1802.
If you are all connected to the same router then port forwarding is not relevant as this applies only to connections coming in from the Internet, and each port can be forwarded only to one PC. Port forwarding is only needed if you are playing over the Internet and there is only one PC connected over that ISP connection.
Assuming your PC can access the Internet at your friend's house, then you should check the gateway address your PC is getting and compare it with the gateway you friends' PCs are using to access the Internet. If they are different, then you are connected to a different subnet, which explains the symptoms you are getting.
I suspect you already know how to find the IP address. The gateway address should be listed within the same set of results.
I hope this helps.