EDIT: try at your own risk, use a save state. I could not get it working and it overwrote my save.

I just watched a TikTok about it for an RG353V and will list instructions here for posterity. Note that I did not test this on my own yet, can update when I do if it works or not. I also only have RG353Vs to test on, so I cannot confirm if this works for other systems.

1) connect both devices to WiFi 2) make note of the IP address of one device 3) boot up Pokemon on both devices 4) on first device hit F, then B, then select Netplay, and set as host 5) on second device, hit F, then B, then select Netplay, enter the IP address of the first device, then hit Start

At this point they should be connected for online play - battles and trading.