The Radius' pitch tracking center note is middle C, so if you play middle C while adjusting the FREQ knob, then the carrier frequency should track up and down as you play.
Here's a simple way to tune it:
1. Set tracking to 12:00 (off)
2. Play middle C while adjusting the FREQ knob to taste
3. Set tracking to 5:00 (100%)
Now the carrier frequency will track notes that you play up and down (monophonic).
An alternate way to tune it is to you the push-to-tune function. Here is how to get suboctaves::
1. Set tracking to 5:00 (100%)
2. Hold the left footswitch (default assignment is push-to-tune)
3. While holding the AUX switch, play C below middle C
4. Release the footswitch
This Instagram reel shows an example.
Once you get comfortable with the tracking at 100%, you can try other settings like 33%, 50%, 67% or negative tracking - which makes the modulation more extreme as you play lower notes.