Yes, you have to manually edit the values in /etc/inc/radius.inc. They're easy enough to spot - if you're not comfortable with command-line just use Diagnostics > Edit file.
Look for a line like the following…
function addServer($servername = 'localhost', $port = 0, $sharedSecret = 'testing123', $timeout = 3, $maxtries = 2)
I've not yet got to the bottom of my problem - increasing the timeouts has not fixed it for me. Unfortunately I've had little time to look into it further, and it's always in use so access is tricky.