I have several AR750S. This article and another helped me to create a workaround for the admin password not working, if the backup config is applied to another router of the same model.
- Create the backup of the source router.
- In the backup file, edit /etc/config/glconfig
- Find section: config service ‘general’
- Under that section delete lines:
option language ‘EN’
option password ‘blahblahblah’ - Save the file and update the archive.
- Restore the backup config to the other routers.
- Connect to 192.168.8.1 and set the language and admin password. All other settings should be intact