"Could not sign in" when i try to sign in to App store with valid Apple IOD and valid Password.

My iphone has always worked perfectly in terms of auto updates to my Apps. Setup was such that if I wanted to download a new app it asked my for my Apple ID password. Fine.

Now, since around 5 February none of my Apps with updates outstanding have updated, and I cannot access App store.

In 'Settings' it tells me iTunes & App Store are 'Off'., but then will not let me sign in to the Store. When I try to 'Update' one of the 6 updates outstanding it asks for Apple ID and password, which I enter, and it just returns me to Update again and again as if in a loop.

The odd thing is that all is fine on my iPad, and on iCloud on my Windows 10 PC with the same Apple ID and Password. Equally on the internet via any browser I can sign onto my Apple account using same ID and password.

I have also tried changing the password for the Apple ID and successfully used the new one on my ipad, windows etc AND on the iphone for FaceTime, imessage, email etc etc. It's just the App store that is "off"!

Can anyone help please ...?