try changing to a different header to see if its just a header ban....or dumping a game with online features like mario kart/smash bros etc and rename to .3dz and see if it will play online with a header that matches that specific game
pretty sure 002-0123 was the code given for sky3ds users when sky3ds botched the template file up....just make sure you have the rom named to .3dz and that the card size chipID etc is all set up correctly
see sky3ds users getting 002-0123 back in December
https://gbatemp.net/threads/how-to-editing-online-header-template-file.377314/