The problem could be how your SSL certificates are setup, in some cases you need to add additional settings for it to run on every known mobile depending on whether you’re using one of the free certificates like comodo or let’s encrypt.
Also SNI may be an issue for older mobile OS’s and browsers.
You could use this site to test it:
https://www.ssllabs.com/ssltest/analyze.html
But AFAIK this can happen for when accessing it using a browser on mobile, not sure if this same case can be applied to mattermost app.
After you run the above test scroll down to handshake simulation and you will see which devices support the current certificate you’re using.
In some cases simple changing your nginx settings as follow can fix the issue:
ssl_ciphers "EECDH+AESGCM:EDH+AESGCM:ECDHE-RSA-AES128-GCM-SHA256:AES256+EECDH:DHE-RSA-AES128-GCM-SHA256:AES256+EDH:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA:ECDHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES128-SHA256:DHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA:ECDHE-RSA-DES-CBC3-SHA:EDH-RSA-DES-CBC3-SHA:AES256-GCM-SHA384:AES128-GCM-SHA256:AES256-SHA256:AES128-SHA256:AES256-SHA:AES128-SHA:DES-CBC3-SHA:HIGH:!aNULL:!eNULL:!EXPORT:!DES:!MD5:!PSK:!RC4";
The above will give u an A+ rating.
Keep in mind that, this is a very specific case scenario with SSL and might not be your issue, this is just me assuming without enough information provided from your side.