The direct way to increase the coverage distance is to manually change the Transmit power to higher value. But this can lead to unexpected behavior.
The indirect way is to allow lower bit rates such as 12mbps and 6mbps, so that farther devices also get connected but at those lower speeds.
When you force the AP to reach longer distances, consider also the client should be able to talk back at those longer distances. So, if your AP placement is already at optimum position, think of doing a plan to consider movement or addition of APs. A Cisco Meraki partner can help you with doing an ekahau based site survey to recommend changes.
For an outdoor AP, like MR76 and assuming you are currently using omni-directional antennas (probably MA-ANT-20), you could consider using a directional antenna / antennas instead, such as MA-ANT-25 or MA-ANT-27. Remember that a directional antenna effectively redirects the same RF energy in a specific direction, so you will sacrifice some range in directions other than that in which the antenna(s) is/are pointed. Also, the increased gain of the antenna will affect the maximum Tx power at which the AP can transmit, due to regulatory limits around EIRP. The net effect though will still be increased range in that direction. Note too that such antennas also provide benefits in being able to 'hear' the clients at greater range, even though nothing has changed in their own capabilities (think of the antenna like a speaking trumpet - it allows your voice to carry further, but if you hold it to your ear also allows you to hear further too. All assuming you point it in the right direction!)
The minimum bitrate you choose is the bitrate your wireless management frames will be sent at. So having lower rates may increase your effective coverage area it will also cause much more overhead because all your management frames take more airtime.
This is a very old capture I did at my house a few years ago but you can clearly see that beacons sent for each AP for each SSID, 10 times per second can take around 2ms per frame if you are using 1 Mbps as minimum bitrate.
The one frame you see in this capture that has a minimum of 12 Mbps only takes 176 microseconds (0.176 ms)
Further to @GIdenJoe 's very valid point, lowering minimum bitrates can also affect the general user experience, simply by allowing some clients to connect at low rates; such clients take up so much more airtime to send/receive their data - time that's not available for other clients on that channel. This is particularly a problem if you have 'sticky clients', who insist on remaining with their current AP, even though they've moved nearer to a different one.