So does this not work with 9.5.0-23U? And if so, is there anything I can do to make it work?
Nope
Depending on how badly you want to edit your town, maybe you can find a friend who hasn't updated, or buy a used un-updated 2DS for cheap or something, but if you downloaded the .23 update you're out of luck, ESPECIALLY if you're on a digital copy. (I actually had to update my primary 3DS, so I copied my digital save and moved it to a cart with Save Backup/Restore before updating)