Comment by miedpo

Pretty sure ZeroTier supports relaying (I remember reading some of their earlier blogs and it mentioning something to that effect). In practice, you just have to turn off the uPnP in Settings to use it I've found.

Edit: Yep, just found a reference to it: https://docs.zerotier.com/zerotier/troubleshooting/ (Sorry, no direct link, so you'll need to Ctrl-F and look for Relay)