Yes, the TPG provided device is a dual band modem/router.
Did you change the name on both 2.4GHz and 5GHz network? Or just the 5GHz only?
Have you tried to use the WLAN/WiFi button on the modem/router if it's going to activate both bands?
If you can't connect to the network after the changes, please forget the network first (remove from the auto connect list), then reconnect to it again. If it's still not accepting the password, then you may need to factory reset the device by pressing the button inside the pinhole for a few seconds.
Once done, you may check if you'll be able to detect both bands and try to change the settings again.
Let us know how it goes.