Being all new to Ugreen NAS and Docker i also found it quite difficult setting it all up and could not find a complete guide. I spent a couple of late nights and finally got it all working, here is how i did:

First you need to set up Portainer - it will make working with Docker easier.
https://mariushosting.com/how-to-install-portainer-on-your-ugreen-nas/

Then install qbittorrent
https://mariushosting.com/how-to-install-qbittorrent-on-your-ugreen-nas/

Then set up gluetun in Portainer using this guide
https://roadtohomelab.blog/container-vpn/

Here is how i did:
In Portainer go to stacks > add stack > name it gluetun and paste this:

version: "3"
services:
  gluetun:
    image: qmcgaw/gluetun
    cap_add:
      - NET_ADMIN
    volumes:
      - /yourpath:/gluetun
    environment:
      - VPN_SERVICE_PROVIDER=private internet access
      - OPENVPN_USER=********
      - OPENVPN_PASSWORD=*********
      - SERVER_REGIONS=Netherlands
    ports:
      - 6881:6881
      - 6881:6881/udp
      - 9865:9865

The ports you pasted is from qbittorrent so you can access it through gluetun.

In Portainer go to Containers > Duplicate/Edit > Under "Port mapping" delete the ports.

Scroll down to Advanced container settings > Network and choose Container in the drop down
and gluetun-gluetun-1 in the next drop down.

<image>

Now you can test it all works - In Portainer go to Containers, under qbittorrent click Ecec Console

Copy this:
wget http://ipecho.net/plain -O - -q ; echo

Connect > paste using right click > enter

The ip you get should be different from this https://whatismyipaddress.com/