To anyone reading this, if you had the same problem as me this might help.
My problem was I could see the basic screens on the modem home page but not the detail of the WiFi networks or detail of any attached devices. I could not change the radio password or turn them on and off etc.
What I found seems to have fixed it is to turn off every single device you have on the network, WiFi and ethernet, also any IOT devices like smart blubs, smart locks etc.
Once everything is off, restart the modem. I actually factory reset it for good measure but anyway when it came up and things connected, I could see them properly and I could go to the wirteless settings.
Previously I could not see wireless settings or any of the devices on the network.
I think this happened because when I first for the modem I simply took the old one out and popped in the new one without turning off all the devices. There were just too many. I believe this meant some clashes of IPs etc in the modem which it couldn't resolve and gave up.
The problem was my own. Setting up a new network (different modem and service) I should really have powered everything of so they all started up correctly. My bad!
PS, the TPG support people were very helpful but I didn't have time to spend with them, so I fiddled around on my own. I promise nothing with this solution above ... but it's easy enough to try if you have the same symptoms.
