Adventures in port forwarding: Tamper proof Local IP?

Hi all! Im trying to set up a GL router as a wireguard gateway and am comically over my head! Im trying to open the port, and the router has an Ip of 192.168.8. whatever , however when I go to add this IP to the port forwarding rule. I see that he "local ip" option is very constrained : 

by forcing my ip to be in 192.168.0 I seem to be prevented to port this device. Is this intentional? Is there a way around it? 

There appears to be a way to set the IP of the device Im trying to port forward but that seems a whole other can of worms. If anyone can give me a tip here, id appreciate it!