How do you call the Nextcloud server (address)? - Is a ping < nextcloud server > successful, while VPN is up?
Did you add the Nextcloud server’s address, you are calling, to config.php?
What does the Nextcloud and the webserver’s log tell?
Unfortunatelly you did not fill the support template with the necessary information.
Did you enable port forwarding in your router th the Nextcloud instance (ports 80 and 443).
Did you take care, that your VPN and Nextcloud work in the same IP-range/subnet inside your private network?