Shielding an RF module from noise
It could be the RF module location near the heat sink.
It could be noise.
It could also be a noisy +3v3 power supply to the module, or heavy current from the stepper driver lifting the GND at the module, thus changing its logic threshold and/or latching it up or crashing its logic chip.
It's on a mezzanine connector, so you can try various approaches to do a differential diagnosis, isolate a cause:
Put a longer connector.
Put a cable between the board and the module and move it around.
Cut the GND and +3V3 from the cable and connect them to GND and 3V3 close to the micro so it uses the same references as the micro that it's talking to.
Measure noise between GND at your micro, and GND at the RF module, also noise between +3V3/GND on the module.
Solder a big decoupling cap on the module.
Etc.
You may find that is is not about location, but a grounding problem, or something else.
If it really is a location problem, and the module works perfectly if you put it at the end of a ribbon cable further away from the stepper driver and 12V input, then this is your fix, and you can ship your board, and brag about how you added that feature &quot;to let the user place the RF module where it gets best reception&quot;.