I use Square with my Ellipsis 8 (rarely, but it does work). Do you get an error message? What happens when you try to run a CC or debit card through?
As far as USB Host compatibility, when I use this app: USB Host Check - Android Apps on Google Play it appears this functionality is not present, or set up out of the box. Enabling it, or getting it to work, would likely require tech skills and procedures that we can't talk about here.