Nextcloud-App for Dyndns-Servers?

Hello. I installed a Nexcloud-server using Diet-Pia and it´s working fine. But I am stuck using the Android-App for Nextcloud: When I enter the URL of my server the App can´t find it, I have to use a browser to access it.

The link to access the server from outward is “https://(DynDnsName):666/nextcloud”.
What link do I have to use in the Nextcloud Android-App? (The 666 is portforwarding from my router, a FritzBox to Port 443 of my RasPi.)