iOS Roon on only one Apple product can't find Roon server

I have 5 Apple products that I control Roon with. 2 x iPhones, 1 x iPad, 1 x Macbook and 1 x iMac. All have latest software and all have Roon on them. All are connected to my QNAP NAS running latest Roon core.

One iPhone runs a little while then stops being able to find the Core. All other devices are just fine.

I have backed up, restored, erased completely and started from scratch, yet after a day or so of it all working fine, goes back to that state.

Any ideas??

Being in Kenya obviously reduces the “pop down the apple store” idea…