Wrote the solution 4 days ago in this post.
viewtopic.php?f=6316&t=136891

Another thing is that i find the heading in this tread misleading since this is only an alternative way to connect to multiplayer.

Hi We have been testing Multiplayer in P3Dv5 to no avail so my conclusion is that Multi in P3Dv5 dosent work unless your on the same network. I might be wrong but this is my experience atleast. The solution for that is simple. Download this tiny vpn app for free https://www.radmin-vpn.com/ then one ...