SSH in into the devices (you may need to enable SSH in the Qnap settings).
Find out the system volume path:
getcfg SHARE_DEF defVolMP -f /etc/config/def_share.info
Go to the tailscale package directory:
cd /share/CACHEDEV1_DATA/.qpkg/Tailscale
Authorize the client:
./tailscale -socket var/run/tailscale/tailscaled.sock up
Or:
./tailscale -socket /tmp/tailscale/tailscaled.sock up
The second one worked on my device.