I'm also testing the netplay functionality and i am not able to get it running. I have 2 PCs with Recalbox 18.06.27 (x86_64), both with identical settings and roms and connected to the internet.
When i start the host on the first PC and try to join from the second PC it always says:
"Failed to initialize netplay."
I tried with catsfc and snes9x (no snes9x_2010)
I scraped and hashed all my roms
And in Netplay Lobby i get the green Ok result (Rom and core match).
But still it does not work ("Failed to initialize netplay" on the PC that tries to join the host).
++UPDATE++
Problem solved! It was the UPnP-Settings in my Router. I activated UPnP and now it works 🙂