Chris,
You are right, that setting applies to EU868 only. You can just leave it out. It should not affect the reception of your messages by the gateway.
Your MultiTech gateways support 8+1 LoRa channels, and maybe the RN2903 is using all 72. You need to disable all the channels on the RN2903 that are not part of the sub-band configured on the MultiTech.