iPhone error "Unable to connect to network"
Whilst I have no idea what caused the problem, it was solved in this case by 


Disabling security on the Wi-Fi network, 
Connecting the devices to the unsecured network (which worked), 
Re-enable security, 
Connecting the devices to the secured network.


Somehow, this solved the problem and all devices were able to connect normally.

On our TP-LINK router, a reboot was required whenever the security settings were changed, i.e. after steps (1) and (3).

Rebooting the phone's was not required, though I think I may have turned the Wi-Fi on the phone off and on again.

Hope it helps!