Unable to connect internally to NAT target(s), no response.

We 've been making a multiplayer game the users can create a server that is registered to the Master Server. It was working fine for a few months now, without any problem. Now we have this problem (see title) when i try to connect to the server that was created on another pc.But if i create the server on my pc and i join the server from this pc also, it works. Our networking script is the same as this tutorial's Multiplayer Script.link to the tutorial I 'm sorry if i wasn't too specific but my English aren;t that good, neither my knowledge in Unity Networking.