Juan, first of all, Yes you can read ebooks on 6 devices by default.
Now, if you are able to read ebooks on your computer, it means the issue is not with Adoeb Digital Editions. Its either an issue with those ereaders or with your device.
Lets do this :
> Make sure your AOC Breeze tablet is connected to internet, go to Google Play and install Bluefire reader or Mantano reader on your device
> Once installed, launch it and try to authorize with your adobe id/password and then check.
If it still say "failed to sign in", contact you device vendor.