Mattermost comes with its own webserver integrated, so when you start the Mattermost application, it automatically spawns up a webserver on the configured ports.
Let’s postpone the installation of a reverse proxy in the first place, would be interesting to see what’s going on with your installation.
Are the Mattermost web apps working fine and it’s just the Desktop App that is affected by that? So if your clients try to access https://your-mattermost.domain, can they connect to the server just fine or does this also not work?