If it's a TPG supplied modem you won't need to save a config because a reset will just reset it to the same state that you received it, the only thing you will need is your username and password, it will load an auto config when it reboots.
Before doing a reset I took it into work for our network tech guy to look at it. He used a signal analyzer phone app that showed that no WiFi signal was being emitted from it, even though the WiFi light was on. We then got into the WLAN settings and he tried changing the channel from 1 to 8. The signal then came up on that channel. Then he went back to channel 1 and it came up on that channel. So it seems that for some mysterious reason the modem was in a bad state but changing the channel nudged it back to normal. He advised me to put the channel on Auto, so the modem will select the best channel available for the area.
So I have my WiFi back. Thanks for people's time and suggestions here.