Q: Can't sign in to App Store

After a total reset (erase all content and settings), I'm unable to sign in to the iTunes & App Store. When trying to download any app, it show the sign in prompt, but after using any valid info (i've tried with 3 Apple IDs so far) nothing happens, the "rotating circle" just stays there and nothing more happens. If I go after that to Settings > iTunes & App Store, the Sign In button is greyed out. If I restart the iPad, the button is enabled again. When I try to sign in from Settings > iTunes & App Store > Sign In, the prompt shows up, I input my info and after pressing the Sign In button, both Sign In and Cancel are greyed out and have to close the Settings app.

I've been searching a solution for this the entire day. I've already tried doing another total reset, restoring from itunes, restoring from backup... nothing really works.

It's an iPad 4 (fourth generation, iPad with Retina) WiFi with 16GB of storage, currently running iOS 9.2