I have setup my arr stack with qbittorrent using i2p.
unfortunately, it never get to download anything. The i2p part seem to be working, but torrents never get downloaded. As a total noob on it, any advices on how to setup it correctly?

If you haven't yet, Try with a well seeded Linux distro, or similar. That will give you an idea if it's the torrent itself or your setup. Without knowing the details of your attempts, verify the torrents have seeds. I think qbittorrent has a tab for checking the status of the trackers as well.

I tried. It's getting a timeout. I was planning to use i2p, but I guess the mixed mode is not working correctly. Well, I guess 'im going to try airvpn or something that is torrent-friendly

You may want to update the post with some setup details, but offhand:

• Did you configure it in Mixed Mode (clicked checkbox) to torrent in both I2P and clearnet? Or you left that unchecked to keep it in I2P only mode? There are less peers in the I2P network so you will encounter more public torrents without I2P seeds, that is normal. Sometimes people prefer it in Mixed Mode for that reason but it does mean you're still downloading/uploading without I2P involved.
• qBittorrent does not support DHT in the I2P network. For any torrents you add in qBittorrent you must also add working I2P trackers to them so they can at least attempt to find peers within I2P, otherwise nothing will upload/download.

To properly test and maybe narrow down issues you could try doing 3 different torrent tests:

1. Download a working torrent that seems to be seeded in I2P, with working I2P trackers on it, and see if it finds peers eventually via I2P
2. Download a working public torrent that seems to be seeded on the clearnet non-I2P, with working clearnet public trackers on it, and see if it finds peers eventually via clearnet
3. If you have DHT enabled: Download a working public torrent, that seems to be seeded on the clearnet non-I2P, with 0 public trackers on it, and see if it finds peers eventually via clearnet

PS - You can review https://github.com/ngosang/trackerslist for a good reference on available public and I2P trackers. There are also public I2P tracker lists floating around here on Lemmy in older posts if you search around