Zerotier shows offline, port 9993 is open?

Im currently trying to add a Ubuntu Machine to a Zerotier network.

I have installed Zerotier on the Ubuntu System as instructed.
The port 9993 is open when connected to a VPN.
Tested via:

time nmap -p 9993
sudo zerotier-cli info

still shows the machine as offline.

I am not sure what else I can try.