In the Solution:
Follow the installation guide to set up your WebSocket port properly. (https://docs.mattermost.com/install/prod-ubuntu.html#set-up-nginx-server)
Speak with the owner of any other proxies between your device and the Mattermost server to ensure wss connections are passing through without issue.
But there is no information about the particulars of the websocket connection. Is there a special port? Something I am missing?