Nighthawk M1 password

Just got a Nighthawk M1 LTE Mobile Router off eBay for use whilst travelling. It is branded Telstra (Australia) and is unlocked. Inserted my AIS (Thailand) 4G SIM card and it seems to work fine. Computer and phone can connect OK. When I use the router power button to scroll screens it displays a username and password (on the SSID screen).

Downloaded the Netgear Mobile App on my Android phone. Not able to get past the sign-in because the app does not permit me to change the Username from "admin". I tried using "admin" with "attadmin" (but I guess that only works with devices bought through ATT in USA). Also tried "admin" and the password stated on the SSID screen. No good.

Tried using the web browser interface on my PC, no good, same problem. Tried tethering the USB cable to my PC. No good.

How can I update the username and password on this device?