H21LOB? Gen7 used the H21LOB up to two weeks ago, so simply roll back a few versions and there you are. The "gen2-3compatible" one still uses the H21LOB and just differs in pin layout of the connector.
For testing an endstop, a voltage meter is sufficient. You want close to 0 volts in the untriggered case and close to 5 volts in the triggered case. Maximum for Low is 1.5 V, minimum for High is 3 V on ATmegas supplied with 5 V.
Firmware can compensate for an inversion, 5V/0V instead of 0V/5V.
You likely also want to get the LED right, it should light up only with the endstop triggered.