You may look into adding custom firmware to it and that may solve your issue. I know when I added custom firmware to my 3ds I was able to use old DS flash cards that never worked on a 3ds. Custom firmware may allow your action replay to work again.
I was going to just leave my DSi on the firmware that worked but, accidentally updated it at some point...not sure when.
Everything I've read online sounds very complicated. Formating the system did nothing. Everything else sounded like soldering and/or custom firmware was the only solution.
Not sure if Codejunkies ever offered an update to the DSi action replay but, if they did, it's no longer on their website. In addition, I've lost the cord that came with it. They can still be bought on ebay, though, for like $10.
Jon