All i can say is that i've had no such issue with my WD My Book World NAS drive over DLNA, after setting up my BDT500 it registered very quickly and i've had no issue accessing it through the player. Have you tried the Wi-Fi direct option of connection? You need to have the I.P address of the NAS and the name of the folder you want to access but it may be an option, if your NAS has a password set then you will need this to but it's worth a go.
Other than that if your Synology NAS is recognised by your computer and network then it may well be the case that it's simply not supported by the player...worth a call to Panasonic for advice imo.
Sorry i couldn't be of more help Chisy1, it must be very frustrating.