I found an old R4 today and it works fine on my DS lite but it won't show up on my 3DS. I remember getting a whitelist file to allow me to use my acekard so is there something similar for this I can use as well?
You should be able to use NTR Launcher to run it.
GBAtemp thread: https://gbatemp.net/threads/ntr-lau...ssic-ds-boot-screen-for-your-ds-games.440110/
Latest release: https://github.com/ApacheThunder/NTR_Launcher/releases/latest