So just to test out, I used an older phone where the region (for the app store) was still set to the US and the set up worked! So if anyone is having similar issues, it looks like the TP-Link device...

Hi, I was using a a pair of HS100 smart plugs in the US with the Kasa app (iOS) with no problems. I recently moved to Japan and was re-setting up the device. I'll be able to connect to the HS100's...