If you have no control over firewall/router/ports config, then I suggest to reach out your client support to ask them about this exact issue, tell them that your server needs this port to be available in order to have the weekly game mode working. If it suddenly stopped working without you doing anything (and I don't see what you could have done honestly), then It may be a problem on their side regarding the communication on this port not being able to work (the server needs to check the external time server to check which weekly it must run so I suppose this is most likely the problem).
edit: wait I read again, you have a dedicated server so you should have full control then. Maybe share a log to see if we can see something weird in it (remove any sensitive info of course like passwords in command line)