A very dirty and hacky alternative may be to try to install a chroot environment with some distribution like Debian and try to install Zerotier in that environment, all via shell without involving packages. There were old guides on how to install Debian chroot environments on Synology devices (in pre-Docker era). Maybe you can still find them if you search.
However, this would be hacky and you can face problems that may require advanced knowledge to be solved or may be unsolvable. If you feel brave enough, give a try.