Keep in mind that the MG51 is not certified for vibration or mobile environments, mileage may vary.  ... View more

That would not work well at 20m+.  10m would be much better.   I would also consider the MR76 which can use external antennas.   https://documentation.meraki.com/MR/MR_Overview_and_Specifications/MR76_Datasheet  You could consider using MA-ANT-25 or MA-ANT-27. https://meraki.cisco.com/lib/pdf/meraki_datasheet_antenna_dual_band_patch_8dBi_6dBi.pdf  https://meraki.cisco.com/product-collateral/dual-band-sector-antenna-datasheet/?file    ... View more