From what you describe I don't think it is the diode (SD1040CS) but let's eliminate that to be sure. With the board powered OFF and your meter in diode mode, measure the forward voltage drop across the diode by placing the meter's positive probe on one of the small legs (either of the outside ones, not the short stubby pin in the middle) and the meter's negative (COM) probe on the large flat part near the output connector. It should read something like 0.3V or less. Then reverse the leads of your meter (positive probe on the fat part and the negative probe on one of the short legs). You should meter should show OL or something similar to indicate an open circuit.
If that passes, then check the frequency on the gate of the Mosfet again. To do that power up the boost board, put your meter in frequency mode, connect the negative probe to the "Input -" of the module (which should be the circuit's common ground) and put your positive lead on the Mosfet's gate pin. Do you read a frequency?