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.