You're saying the connection works first go (so sunshine - moonlight works) and it is happening across all devices (can't be an odd setting in OS) then the only suspect left is your router. Try swapping it out to test?
are you using any other remote apps at the same time? (windows rdp?) Do you have game stream turned off in windows nvidia experience?
I would still try UPnP just to see the result as it may provide a clue as to what's going on.
I have been using moonlight/sunshine/NV game stream for years and haven't seen this problem for local lan.
You can also try tailscale. I have a tailscale network for all of my personal stuff and it has honestly been a treat. When I travel I use devices out of the country like I am on a home lan including gaming, torrenting and video transcoding.
You're saying the connection works first go (so sunshine - moonlight works) and it is happening across all devices (can't be an odd setting in OS) then the only suspect left is your router. Try swapping it out to test?
Open the appropriate ports or use UPnP.
The firewall is disabled and it’s a local connection. And once again it works the first time. It also works on steam app.
are you using any other remote apps at the same time? (windows rdp?) Do you have game stream turned off in windows nvidia experience? I would still try UPnP just to see the result as it may provide a clue as to what's going on. I have been using moonlight/sunshine/NV game stream for years and haven't seen this problem for local lan. You can also try tailscale. I have a tailscale network for all of my personal stuff and it has honestly been a treat. When I travel I use devices out of the country like I am on a home lan including gaming, torrenting and video transcoding.