Post one here. So can I check if they display OK on my device. Then it is most likely a problem with the font in your device. Should be easy to add a custom font.
Or even simpler: Just check the epub above in your reader. Can you read all chars? If you can't then it is a problem with your device.
A custom font that include diacritics should do it.
Ask on the Onyx forum how to add a custum font, if it's not in the manual.
https://www.mobileread.com/forums/forumdisplay.php?f=220