I got an answer for this issue from Juan Leyva
Juan Leyva added a comment - 2 hours ago
unfortunately, we cannot accept self-signed certificates in the app or provide a switch to enable them. Please see a detailed explanation here https://cordova.apache.org/docs/en/latest/guide/appdev/security/
What's the point of using a self-signed certificate in the MoodleBox for a local network? I'd suggest that in that case it would be just better to use HTTP
Ralf Krause added a comment - 1 minute ago
yes, the MoodleBox is a server device for a local network. The idea with the set-signed certificate is that modern browsers want to get https connections. In the case of http only some browsers are showing a red coloured "not secure" error in the address field or a not secure message before opening the website. If you import the self-signed certificate into your device you do not get those messages anymore. https://moodlebox.net/en/help/connexion-https/
But if we setup the MoodleBox to https with the self-signed certificate then we can't use the Moodle app.
Best regards, Ralf