If you go into the wifi information on IOS, does the phone get a valid ip address, dns etc. ?

Do you have bind IP address to mac, turned on, on the router ? If so try turning it off, as a trial and see if the problem goes away.

Can you ping the router, from the IOS device , and the IOS device ip from the router? If you can what happens when you try and ping 8.8.8.8, for example ?

We've bound IP addresses because otherwise we did not get an IP address at all (just a 169.xx address). Likewise turned off 'private IP address' on the Iphone for the same reason. The AP 910s in particular seem to be problematic on this front - to the extent that I am reluctant to buy more Vigor APs in the hope that a newer one fixes the issues. We bind IP addresses because we have 2 WANs (1 business and 1 home) and route certain devices through one or the other.
The firmware after 4.4 messed up Vodafone wifi calling (although other wifi calling on the same phone [dual esim] worked ok). Draytek said to roll back 4.4.1 to 4.4. I have not tried 4.4.2 yet.
Can't ping the router. It's as if we have a connect to the AP, but not beyond. AP logs do not show connection. The AP's are connected to Netgear POE switches - one which is connected to the router, the others through a 24-port Netgear smart switch which is connected to router. Of course, the villain in all this could also be the Netgear switches.

Recent update :- I turned off all roaming assists (fast roaming, RSSI thresholds and Band Steering) on the router and APs - this has improved the situation slightly. I'll see what happens in the next few days.
It does seem that the Vigor (or Netgear) roaming systems are in contention with the Apple connection systems.

Sounds as if your dhcp service is not working correctly, you shouldn't need to use bind ip to mac to get a mobile to get an ip address.
If you manually set a valid ip address, gateway, dns, etc. on an IOS device. Ideally one not in the bind table. Can you then ping the IOS device from the router and visa versa ?
I take it you are using the router to do dhcp service, not the APs ?

Colin

Thanks for the ideas on this - much appreciated.
Correct on the router for DHCP server - and this is not enabled in the APs, and the switches are just running in 'simple' mode for now (previously we had VLANs etc. but this is all disabled now).
I'll try manual setting and pings. I'll also double check the DHCP flag in the APs (I'd never set that to act as a DHCP server, but I have not specifically checked that it's not been ticked by mistake)

Doesn't iOS use randomised MAC addresses anyway? So, binding IP to MAC would be pointless long term, anyway. As the others have said: you should switch this option off.

Private IP is switched off. I’ve been experimenting with different settings but still intermittent failures. This week I had an IPAD and an IPhone connected to the same AP at the same time - iPad had access and iPhone did not. Cannot see the router from the phone. I could see both devices as connected when looking at the AP with the iPad. It’s as if the MAC table is not being updated properly (in the router and intermediate switches) when a device changes AP.
Next up is to compare the router ARP cache with the switch MAC tables.