Hi EdgeFarming,
In your scenario, with an MR76, it is an acceptable option only to install the 5 GHz antenna. The requirement would be to ensure that the unused antenna connector is covered appropriately.
Our FAQ documentation covers this scenario in more detail and can be found at this link - Frequently Asked Questions for Cisco Meraki Access Point Antennas
Specifically, the sections named 'Can I leave any AP antenna connectors unused?' and 'Can I leave unused connectors uncovered?'
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