Unfortunatelly you ignored the support template you removed while typing in your posting above.
I will try to guess, what you did not communicate with us:
- Nextcloud Android app 3.30.7 is buggy, so install 3.30.5 from F-Droid
- In normal cases Letsencrypt should be installed on Nextcloud server for access from internet. If you want to use Nextcloud from internal network only, you will have to assign a static IP address to Nextcloud server and add this IP address to config.php
- To syncronize calendar and contacts you will need the app DavX5 from Google Play store. While configuring it, take care to mark “Do not validate certificate” or similar.