In the video Russ uses JELOS on a Powkiddy X55, RG353V, RG552, and the Ayaneo Air Pro to show how to setup net play and provides a few examples. He states that he wasn’t able to get Pokemon battles/trading working. GB games work when a save game isn’t required.
It seems like a limit of netplay or the core. People apparently use something called Parsec and Retrobat or Steam’s Remote Play Together.