Disconnecting Apple Books from Amazon Japan Account

My Amazon Japan and Amazon US accounts are treated as two separate accounts, and I have long ago logged on to the Japan account to access a kindle via Apple Books. But now that I try to access my audiobooks from my Amazon US account, and it asked me to authorise my account, it kept going to Audible Japan and didn't give me any option to disconnect my apple account from Amazon Japan and reconnect it to Amazon US. I can't do anything on Audible Japan's website either, since it's all in Japanese (which I cannot read). Does anybody know what to do in this situation?