I am downloading through Mullvad, which I know doesn’t let you forward your ports. So I can appreciate that that seeder’s settings and mine might not be super compatible.

Are there any other peers in that torrent swarm? If it's just you (leeching peer) and the lone seed (seeding peer), and neither of you have ports open, then you won't be able to download any torrent data.

If there is another peer in that swarm, or another peer joins later, and that new peer happens to be fully connectable (port forwarded) then you'll be able to download the torrent data through them. If this is your situation then all you can do is try your luck and wait for another peer to come by.

Or if we rule all of that out - it's possible that lone peer just has a very busy torrent client. They could be the lone peer on tons of other torrents so it would take quite a while before their torrent client gets around to sending you torrent data. If this is the case then it's the same as above, just have to continue waiting.