Kirk here. Chay posted this inquiry about where to get help with a OnePlus phone. I found the following shop in Hikari no Mori:
https://s-phone.jp/kumamoto-yumetown/?item=oneplus#price
I wonder, though, if anyone else has any good ideas about places to get smartphones looked at. If there’s a good little shop where someone there can also speak English that would be ideal but I suspect that technical expertise is probably more important than communication skills for Chay.