trying to play with friend on a private server

[plucky latch]

with some friend we have been trying to open a server to play, we used steamcmd the problem is that sometimes we can see the server on the game on listen server and play, but other times the server doesnt show up and we cant do nothing, we always do the same to open it so i cant think what could be the issue.
we have read all the guides, open all the port that guide told us and nothing

[warm arch]

you could use something like Radmin or Hamachi

I would recommend Radmin as it allows alot more people to connect to one network