Is it possible to use Radius as primary authentication server &
Firebox-db as secondary authentication server which will be used
if the radius server is unreachable.
It is possible to use alternate Authentication servers for most features, but you generally need to specify that -- it's not automatic.
-If you're using the web sign on a webpage (like the 4100 authentication page) you can specify the auth server via a dropdown menu.
-If you're authenticating via SSLVPN, you can specify other servers that are enabled by putting the server in front of your username. You can see examples about halfway thru this article:https://www.watchguard.com/help/docs/help-center/en-US/Content/en-US/Fireware/mvpn/ssl/mvpn_ssl_client-install_c.html
WatchGuard Customer Support