If your account is associated via steam (bought the game via steam/have a steam license of FFXIV) then buying regular codes would not help unless you're prepared to abandon your account and start new- as the steam codes and player account are connected now.

Do you get any error or what do you mean by can't do anything when logged in? Like you can see the game, just can't click stuff (it turns on and all, even animates/plays music)? Or is it like a black screen, literally nothing happens?

I don't use the steam version but I've heard some people have issues with the steam overlay and FFXIV, so you could try turning that off: https://www.youtube.com/watch?v=bomISP9pKnI .