First, I recommend to check this article from Meraki document :
https://documentation.meraki.com/MR/Monitoring_and_Reporting/Low_Power_Mode
According to article, it seems your PoE switch doesn't support 802.3at(PoE+),
therefore PoE just couldn't push more power to AP.
Second, if you use external DC power, AP will use only DC power, and tells to switch that it doesn't need PoE.
So, you cannot use PoE and DC power simultaneously.