If your web panel shows Error Connecting for your game server when it is actually online,
ask your web host to open UDP incoming from your game server port (usually 27015).

If either the RCON console, Admin Rehashing or Kick-It does not work on your web panel,
ask your web host to open TCP outgoing to your game server port (usually 27015).

Use this small testing script to get more detailed debugging information. Open the php file and add your gameserver's ip and port as described in the comments.

You should always check beforehand, if your gameserver banned your webserver's ip with listip in rcon. Remove it from the banned_ip.cfg and removeip IP via rcon.