Cannot connect to app store

I have an iPad mini 3 with iOS 9.2 beta. Recently there is something wrong with one of the apps I have downloaded, so I decided to uninstall and reinstall the app. However, after I uninstalled the app, and I wanted to redownload the app from app store, my app store just showed blank page (Cannot connect to app store). So I tried these method below and non of them worked

1. force restart
2. reset all setting
3. reset network setting
4. connect to my computer's itunes (Windows OS)

Moreover, my itunes store on my computer was not working either, showing iTunes could not connect to the iTunes store, the network connection was refused but I am 100% sure I am on an active network.

So are there any other solutions guys? I need your help urgently, thanks!