Unfortunately, Amazon did not design their reading app very well, it apparently took chaley considerable effort to get the builtin reader to work at all and even then the tricks he used will make the reading position reset I believe.
Somewhere in the settings you can control where MOBI formats get sent, so that the builtin can see them.
See here for more details:
https://www.mobileread.com/forums/sho...25&postcount=5
(CC will automatically update according to your Appstore update settings.)