If your box isn't globally addressable (because of NAT), your box can't be connected to. It works one way only, from the inside out - because the NAT-router keeps track of the connections your box makes to globally addressable hosts and forwards reply packages back to your box.

You could use IPv6 which because of the vast amounts of ipv6-addresses, eliminates the need for NAT. Or you could use a VPN or a tunneling service which gives you a dedicated IP. Or port forwarding from a globally addressable host. Either self hosted or as a service. Switch to an ISP which doesn't do CGNAT.

In short: ipv6 is easiest.:)

Edit: does anybody know if a non addressable seed box gets info about interested and globally addressable peers somehow (either tracker or tracker-less) so it can initiate a TCP connection to those peers? Are there resources to read up in that topic?

Unless they let you do some kind of port forwarding, or you tunnel your traffic over a VPN that allows port forwarding, not possible.

I suppose you could also add an http source to the torrent, or set up a seedbox somewhere and preload the content, either by direct upload or by manually adding the seedbox as a peer.

If I create a new torrent, does CGNAT (carrier-grade network address translation) prevent me from being an initial seeder of that torrent?

It's not ideal, you won't be able to seed/upload to any other firewalled (non port forwarded) peers.

But technically any connectable (port forwarded) peers connecting to you will still be able to download from you, so you'd still be able to seed in that sense. That does mean you're only initial seeding to peers that have their own ports open/forwarded.

I can download and upload in my torrent client just fine, so I know my ISP isn’t (intentionally) blocking and firewalling torrents.

Yup that would work fine, you can participate in torrent swarms that contain other connectable peers and that's usually how most torrent swarms are. You'll probably have trouble with the random low seed torrents e.g. a torrent with one lone seed who also has no ports open, that seed won't be able to send you any torrent data.

I know my answer is not what you’re looking for but the other two comments should suffice so

Consider IPv6 if possible