Hey @stephanfriedrich ,
As far as I understand: Yes, you will have to build the mattermost apps on your own.
If you need help with setting everything up, I can recommend joining the mattermost nightly build chat server.
Everyone in this chat server is very helpful and dedicated, give it a try if you didn’t yet!
Mattermost Nightly Build Server
You will need basic app development knowledge though(if you’re used to working with IntelliJ / AndroidStudio you won’t have any problems).
I did some great improvements when it comes to app development: I got the dev environment set up on a Mac for iOS and android. 
I wasn’t able though to succesfully test sending out push notifcations via the MPNS.
Thanks to elias (I can only mention 2 users in a post, as I’m a new user, otherwise I’d mention him here.) for providing a work in progress tutorial regarding the setup of the dev environment for both iOS and android!!
I am currently working on a centralized logging server and the year is short, so I do not know if I can start working on the push notifcation again this year. But however: I got “working mattermost push notifications via MPNS” as a objective for Q1 2018. And I’m confident that I will achieve that goal.
@lindy65 closnig this post with “solved” was ok for me, as I honestly gave up on it as written above. But I guess you know that
Cheers