If UPnP devices do not see each other, most likely there are two problems:
Multicasts are not forwarded
Simple Service Discovery Protocol (SSDP) uses multicast IPv4 address 239.255.255.250. The local switches must be able to forward such traffic. On a Cisco switch you run the following command to allow that:
no ip igmp snooping vlan 101
Firewall blocks UPnP traffic
Related ports: UDP-1900 and TCP-2869