Unfortunately there’s nothing we can do to help you there @gronostaj. Synology changed things and packages can no longer run as root. root access is required to change network settings. As such, ZeroTier requires root. The docker package helps us get around that, but doesn’t seem to be supported by Synology on their older platforms. They really left us in a lurch with this change in the DSM 7 update. At this point in time the Docker method is all we can offer directly on the device itself.
It is possible to route to a physical LAN or use a Raspberry PI to bridge into a LAN. Those are ways to be able to route/bridge to devices that ZeroTier can’t be installed on directly.
I personally own a Synology NAS and do like it a lot, but haven’t ever used the ZeroTier package on it myself. I’ve always had a Raspberry Pi bridge into my home LAN so I can access the entire thing while out of the house.