Turns out if you go to the root path /RELEASES you can find all legacy builds which is great and I have found a version for this architecture at: https://download.zerotier.com/RELEASES/1.8.3/dist/synology/zerotier_88f6281-6.1_1.4.0-0.spk
Fantastic!
Ok, so now I install it, package installs, the GUI doesn’t work correctly but that’s fine, I login using SSH and I just keep getting “zerotier-cli: command not found”, sudo makes no difference.
I found this post by @pawol Synology DSM 6.2.4 255556 - latest update - #16 by pawol to chown the /var/lib/zerotier-one directory but unfortunately that has also made no difference.
Any suggestions appreciated.