Iphone 6 does not log in to iCloud, why?

Pe
1

I have an IPhone 6s but for a few weeks I can't log into ICloud with my mobile phone. I enter my data as normal, then it is logged in with ICloud and later it jumps out on the previous page without having logged in.

First thought that I enter my data wrong but when I log in with my laptop everything works. Since then, my cell phone battery will not last much longer.

Have turned on and off and waited a few weeks but nothing changes. All my contacts or similar All were on it but unfortunately have no access dadrauf with my phone. My phone is now about 2 1/2 years old, vlt it is now just too old or similar.

Can someone give me a hint?

Ty

The 6S is not too old for that; I'm very satisfied.

Maybe make a local backup and then reset 6S and restore the backup again.
try with another Apple ID (does not cost anything)
maybe the 2-factor authentication is active and you do not enter the number?
Make appointment in the Apple Store