Try turning the radio off.
Hold down the "F" key, the "Mon" key and turn on the power. You may
hear a series of beeps then release the keys.

I have a C510 and this is how the reset sequence goes for it and they
may be using the same CPU or programming for it.

I presume "F" is the function key.
I cannot find a "Mon" key on this rig. Is it labeled something else?