I tried to configure sync between three devices: synology, win7 and android. Win7 and android shows global discovery ok (1/2), but nas always shows (0/1). Syning between three worked (because of local discovery, I suppose). Both win7 and nas are configured with upnp support, both are showed on router, everything looks fine from that point.
No errors in log on nas:
[4A4MP] 19:08:12 INFO: New UPnP port mapping: external port 7097 to local port 22000.
[4A4MP] 19:25:31 INFO: Established secure connection to at <NAS_LAN_IP>:22000-<ANDDOID_LAN_IP>:50816
Where should I dig from? Why global discovery if offline? I think that it is working only because of local discovery. It would be dissapointing to fail syncing from other network through internet.