So if I understand this right you will need to change the network on the port attached to the synology in your UniFi configuration or set the vlan tag in the synology OS, I would do the former. It sounds like you just added a second network/vlan to the existing interface which means you actually created a trunk and are getting the old network untagged and the new network with vlan tags which the synology is dropping. Synology OS also doesn’t really support trunked ports through the UI (even though it does support a port that only uses a vlan tag) so it’s much easier to just leave them untagged.