Nestcloud is not an own application. It’s accessible via your web server running on port 80. Using wireguard is one option you could use. For Wireguard you would need to forward Wireguard port.
Another option is to follow attached guide to get direct access https://dietpi.com/forum/t/how-can-i-secure-connection-to-nextcloud/3820/4
The DDNS part changed in meantime and you could use our new tool dietpi-ddns or use any DDNS functionality of your router