What radius server are you using which is unavailable?
Have you looked at the logs on the radius server to see why it's unavailable? Is the radius process broken? Can't ping it by ip? unavailable doesn't give us much to go on as to why it's not working.
If you have AD (sounce like you do) you could stand up NPS/IAS as a radius service. I don't really like it but this would be a way to get a radius server up more quickly so you have some trouble shooting time on the other radius server.