Mattermost has a brand new, open source Android client in beta written in React Native used by thousands of testers: https://play.google.com/store/apps/details?id=com.mattermost...
It's greatly improved from the original version and official release is this month.
Mattermost mobile notifications aren't a commercial feature, they are just off by default due to privacy (push notifications appear on a mobile device when the device is locked).
Go to System Console > Notifications > Mobile Push > Send Push Notifications to enable.
You can also deploy your own Mattermost Push Notification Server (https://github.com/mattermost/mattermost-push-proxy) and compile your own Mattermost Android and iOS apps (https://github.com/mattermost/mattermost-mobile) for total control.