If you are getting the 'connection timedout/lost' message and its looping (continually trying to connect over and over) try the following.
Close out Atlas;
In your file manager, navigate to X:\Steam\steamapps\common\ATLAS\ShooterGame\Saved (X being the drive your Steam/steamapps directory is located);
Rename the file "ClientGrid" to "ClientGrid.old;
Restart Atlas, and choose Rejoin Server.
If this does not work, or caused further problems, simply rename "ClientGrid.old" to "ClientGrid"
Should not be an issue as a new (updated) version is generated when it missing.
Worked for me.
Good Luck!