Yup, that’s exactly what it’s saying. “No route to host (Host unreachable)”. Routing SSDP over a VPN connection is a tricky thing. My quick research of the problem seems to point to the TTL (time to live) setting of the SSDP messages. The complications get more interesting since there is also a TTL for the packet encapsulating the SSDP message.
There is also the possibility that the SSDP message isn’t properly reflected back across the VPN from the remote site.
BTW, Wireguard is an exceptionally light weight VPN. It can cause other problems because it is only focused on UDP traffic. It doesn’t like to pass TCP. (Someone has probably kluged a way to encapsulate TCP in a UDP packet, but that would be a very ugly baby. )
I suspect there are others in the community who have much more expertise in this area that can help. (@aaiyar do you know anyone?). I also believe this is such a specialized non-Hubitat issue that a VPN focused networking forum would get you an answer quicker.
I wish I could help more.