iPhone 4s upgrade to iOS 9.3.1 is forgetting text messages

I have an iPhone 4S (32GB). It was on iOS 6. I upgraded it to iOS 9.3.1. It removed all previous text / imessages (that's fine).

But now I am getting text / iMessages late. It's also losing (forgetting / deleting) messages since I've got since the upgrade. Sometimes (most times) It clears all my messages and I don't get them back. I also suspect I am not getting some messages at all (text and iMessage).

I've tried:

- turning iMessage off and on

- restarting the device

- resetting network settings

This is a complete 180 from what I have come to expect from Apple.

I've google for the solution for this, none has been found.