I don't enter any port number. I just give it some really short name (like ua) and click save with the port field empty.
However, I also have to delete and re-add the custom entry every flight.
I'm not sure if it is ADS-B installation issue, iPad issue, FltPlan Go issue or user error, but if I leave the existing "Custom Unit" in the FltPlan Go app, it never connects. The iPad will connect to the uAvionics unit, but the App doesn't.
I found that deleting and re-adding the "Custom Unit" fixes this, and the App connects immediately after I hit save.
So, after start-up and before taxi:
Make sure iPad is connected via wifi
Open FltPlan Go
External -> swipe left on ua (or whatever you named the existing custom entry) and delete, then tap Add custom Unit, enter some name and save.
If this were some paid app I'd be more inclined to go to the vendor for help, but seeing as FltPlan Go is free (free charts, free geo-referenced approach plates, etc) I'm happy to do this simple workaround.
I'd be curious to know if yours connects multiple times, meaning you don't have to go through the process above like I do.