Hi, I wanted to set up UPnP on 19.07.1 but the host (Zyxel NAS326) did not find the upnp capable router. After much troubleshooting I found the reason. Perhaps this is of help for others hitting this issue:
The reason was the following: I have a TP-Link TL-SG2008 switch between the OpenWRT router and the hosts on the LAN. The switch was configured to block unknown multicast traffic. Since UPnP and the underlying SSDP protocol is using multicast (and unicast) packets, and the switch blocked discovery traffic that is using multicast, the upnp host could not discover the upnp capable router on the network. After enabling Unknown Multicast Forwarding, everything works fine.