Can the person from whom you got the device help?

Seems to be a lot of support (and manual) here: https://crowdsupport.telstra.com.au/t5/Modems-Hardware/T-Gateway-User-Manual/td-p/92998

More general stuff here: https://crowdsupport.telstra.com.au/t5/Modems-Hardware/bd-p/Modems

You may get advice there how to do a factory reset

You could do well working out exactly which model you have.

XXXXX
October 2015