Hi!
After much head-scratching I figured it out but for a new player it's a real curveball when you skip over a bunch of relevant details. ;)
Your description/comments say to use the radio nob to select a signal and number... somehow.
What the player actually has to do is:
Click on the round knob/dial part of the placed radio to open the selection window.
On this window you use the logistics interface to select an arbitrary item and a quantity for it.
That item/number combination is a unique channel that can be picked up by all other radios if they are set to the exact same thing.
Now that may be obvious to YOU but I just got the game this weekend, okay? ;)
