If you are a user of ZeroTier, you have probably noticed that it stopped working correctly on the ReadyNAS. It happened to me when I finally updated from OS6.9.3 to 6.9.5. Specifically, no ZeroTier IP address is being assigned. I've been looking into this and, unfortunately, can't give you a fix. But I can tell you it's not unique to ReadyNAS, so I hope it will be fixed soon. Other Debian users have the same issue. Some potential fixes have been suggested, and I have tried them all to no avail. If a fix does come along, it's likely that you will have to replace the existing Netgear-specific app with a generic install via SSH and lose the GUI (if you are still using that version).
My temporary fix is to manually assign a ZeroTier IP via SSH after any re-boot (I use 192.168.0.xxx for local and 192.168.192.xxx for ZeroTier). It will retain that IP until re-boot. I'm not sure if it has to be the same IP ZeroTier would have assigned it (mine is), but it certainly can't be the same as assigned to another device.
I suppose I could add a cron job to do this a minute or so after re-boot, but I'm hoping for a fix before I resort to that. But having to remote in to my brother's PC to SSH into the NAS at his place is a PITA. Fortunately, his power doesn't go down often. At the very least, I should create a PuTTY script for him to run.
Oddly, I temporarily resurrected a Pro6 running OS 6 and ZeroTier, which I promptly upgraded to 6.9.5. Low and behold, ZeroTier still works on it. The investigation continues.
