Mattermost error websocket


I recently installed Mattermost at work, using Nginx as a proxy. We are having a problem when working at home without VPN, once we launch Mattermost, its working for approximatly 20 seconds then this error appears :

Please check connection, Mattermost unreachable. If issue persists, ask administrator to check WebSocket port.

I already read a lot of previous topics about this error but none of them could give me an answer, do you have any ideas of where this could come from ?