Board Thread:Questions?/@comment-121.217.56.143-20130604055133/@comment-3448772-20130604184635

The DSi for the purposes of the online DSi Shop is region locked, meaning you can only access it in the region that the DSi is from.

So, it is not a matter of re-downloading the DSi Shop, it is about the DSi its self. So there really isn't any way of fixing it other than getting a DSi from the region that you are now in.