Cannot log into App and iTunes Stores [iPhone 6]

It seems that every time I tried to log in, it's not giving any response. The 'Sign in' button never reacts, and by the time I opened App Store and tried to log from there, the popup flashes and closes unexpectedly, making me unable to click. I also tried checking the Internet, whether it blocks the Apple server or not but it's still the same when using cellular data. Now, I can only download through iTunes in my laptop. 😟