Looks like @Daisy on the Moto help form that you already have posted on had a reply from Amazon that called for other changes in addition to the App to get it to work. I'd suggest seeing is here reply is enough guidance to get you going again. Good luck.
I have a Moto phone that will not connect to Amazon music on my home WiFi but works perfectly on LTE (3G/4G) and other routers that provides IPv4 and IPv6 connectivity, which allows devices to reach websites and web services with native IPv6 connectivity.
My ISP provides IPv4 connectivity only. As a result my Moto phone on my home WiFi will not connect to Amazon music via the app. It will not connect Amazon shopping app either via my home WiFi.... Strange considering I bought the phone via Amazon store!!!
Is it something to do with how phones Handel IPv6 DNS DCHP and router advertisements?
Most (but not all) other apps work fine on my home WiFi.
I have tried phone resets, router resets, contacted my ISP provider still no change.
I would like my phone to work with Amazon apps on my IPv4 network and router.