Untrusted Certificate on Android Mattermost app

Hello,
I have installed a Mattermost server on my QNAP nas, I have no problem to connect to it in local and external network via a browser. On the other hand, when I want to connect to it via the android application I have an error message: “Untrusted Certificate”
I have already tried restarting the server and reinstalling everything but it still does not work.

Can you help me ?