Some update on my side.
I spend several hours with some expert from support.
It turned out, I have a dedicated public address. It’s not shared. So all should be ok. We tried port forwarding the port 10308, but all checks done on portchecker.co/checking showed that the port is closed. They didn’t understand it, and told me that the port is open.
Now I got it. Of course the port is closed. The port is only existing when I create a multiplayer server. I learned a lot during this exercise! Best training ever!
I’m sorry for it. All is good!
Now: “get to the Choppaaaa”😂
Flanky83's post in AH-64D how to play with a friend without « port forwarding » was marked as the answer