In North America, can I use a high gain ISM LoRa antenna?

The following information applies to the MultiTech Conduit Gateway:
--> https://www.multitech.com/brands/multiconnect-conduit

For 915MHz the maximum allowable ERIP by FCC is 36dBm in the United States. The maximum configurable Transmit Power of the Conduit (with a MTAC-LORA-915 installed) is 27dBm.

Let's say, for example, that we're using a 10dBi gain antenna. The Transmit Power is configured for the maximum 27dB. The antenna gain field is configured for 10dB. The Conduit will automatically attenuate the signal so that the ERIP does not exceed 36dBm. 

To configure these parameters in the Conduit AEP, first log into the web administration with a browser, then select the "Setup" tab, and then select the "LoRa" tab. The mode field allows you to choose between "Packet Forwarder" and "Network Server". The above information was based on using the Conduit AEP configured as a LoRa Network Server. If using the Conduit AEP as a packet forwarder to a 3rd party LoRa Network Server, the high gain antenna question would need to be directed to the vendor of the 3rd party LoRa Network Server.