I can manually edit the EPUB file and add some CSS to refer to font files I place on a folder on the device itself. This gives me the correct characters on the device!
The next step would be to put the fonts into the EPUB itself.
From there I need to work out a way to automate the font embedding as much as possible as editing the EPUB is a bit of a pain, especially given that I tend to re-create the EPUB regularly as I work through creating my book.
thread:
https://www.mobileread.com/forums/showthread.php?t=58901