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).
Problem solved! It was the UPnP-Settings in my Router. I activated UPnP and now it works :-)