> I think zerotier does UPnP too
I think you're correct. Thanks. My information was from an old blog post from 2014 [1] but they seems to have added support since then: https://github.com/zerotier/ZeroTierOne/commit/bf193dd3. Edited my post.
[1] https://www.zerotier.com/2014/08/25/the-state-of-nat-travers...
