There are Wiki pages addressing workarounds, how to, buglist, wishlist, etc.
https://wiki.mobileread.com/wiki/Boox_M92
Concerning your issue. It is possible to select the reader for e-pub and some other formats. It is somewhere in settings. I guess it will be also described in manual.
One does not choose which reader to use at the moment of openning the document. Instead, prefered reader for docs being open may be set in advance (in settings).