Hey, @RbDev! Sorry to hear that subpath didn’t work the way you expected.
There’s a brief note on how to configure for a subpath in our SiteURL documentation:
In Mattermost v5.1 and later, the URL may contain a subpath such as https://example.com/company/mattermost.
There’s also some more technical details on our developer blog (the blog itself still being under development).
Can you test with an updated SiteURL (and restarting your server) and let me know if that works for you?