Hi @tomx,
I think you can reset the password using the swiflash linux tool. You can use the below command which will restore the Linux root filesystem in its original state by erasing the USER1 partition and your password will be reset.
./swiflash -m WP77XX -r
If you have not installed the swiflash , You cna fallow the instructions in the below link to install the swiflash.
https://source.sierrawireless.com/resources/airprime/software/swiflash/
Regards,
Muralidhara N.