My question is about back up on RADIUS server. Is it possible to back up network devices configs on a RADIUS server (automatically or manual)
Radius is a networking protocol, aimed at Authentication, Authorization, and Accounting.
It is not designed to backup configurations.
Now a device that can act as a Radius server may be able to perform other roles, like a FTP server on which a network device can backup is config, especially if you radius server is Linux or Windows machine. But this has nothing to do with Radius itself. It would be two different services that happen to be hosted on the same device.