Meanwhile, if you go to Riot/Web on mobile for a custom deployment, you should see a page like this https://webchat.kde.org/mobile_guide/ which guides you through the process.
Deeplinking from web into the app is hard, as the only way to do so vaguely reliably is by fingerprinting the browser using something like branch.io, which is distasteful from a privacy perspective. Meanwhile, affiliate links in google play seem to only work about 30% of the time, given the planets have to be in precisely the right orientation for the association to not get lost.
Mattermost might be doing something smarter (perhaps letting you install the app generically, and then getting you to click a custom URI handler link from the web to provision the correct config on the client?) - but would be useful to know which of these failure modes people were falling into?