First, according to their site, it does support OTG.
Second, a microUSB adapter with a microUSB-to-Type-C adapter doesn't support OTG - you need a Type C OTG adapter. (The connections are completely different - in microUSB it's just a short from sense to ground, in Type C, it's a 5k resistor from CC1 [which a microUSB plug doesn't have] to ground. So the short wouldn't work, and it's not carried to the far end of the cable, so the phone wouldn't see it even it it would work.)