The difference between UPnP & PF is UPnP within the device opens whichever ports the UPnP capable program asks for. Whereas Port forwarding, you specify a particular port which is used all the time. It tends to be more reliable than UPnP. The other difference is you need to set up a static ip but that's nothing special. If you want instructions we'd need to know the specific model you're using. Port forwarding will only work with a single port per computer per program. So other computers on the network wouldn't be able to use the same port.

Oh Birdy ... that's never happened to me with PF. It's always remained the same listening port.