No internet on some devices

I could do with some guidance on an issue that confused me:

Usually I have a Virgin MEdia Hub 3 in modem mode connected by ethernet cable to an RAX120. For the last 8 months it's been faultless......... 

Two days ago (approximately) the RAX120 stopped allowing an iphone to connect to the Internet. There are 3 iphones in the house, and only 1 was affected. We left it alone, and overnight the other 2 also stopped being able to access the internet (internet light is flashing and the netgear splash page on the router confirms good internet connection). We restarted the system in the sequence as mentioned in the netgear instructions, and then noted that one of the computers also stopped connecting to the internet - we tried this on both wifi and wired connections. 

There were other devices not affected, namely 2 Nest cameras and 2 work laptops that go through a VPN (one on a wired connection to the RAX120, and the other on wifi).

Firmware version was the latest on the RAX at 118, so as it had an update over the weekend, we downgraded it to 114 OK. Same fault remained. 

We then put the Virgin hub back into router mode, and all the iphones could now connect OK directly to the virgin hub, there were no faults anywhere. 

The Ethernet cable between the hub and RAX120 was swapped out (as it runs through a wall maybe it was damaged), put the hub back into router mode, and the fault was back.

The only odd thing is that after the hub and router are reset, for a few moments all devices work OK when wireless connected to the RAX120, but rapidly stop connecting to the internet (the wifi connection to the RAX120 is fine).

Any ideas whats going on here? The virgin hub is in a hopeless spot in the house and can't be moved, so its a bit of a pain having to revert to it. I'm out of ideas why the netgear router would do this!