The main reason devices stay connected to access points, also know as being sticky, is due to low data rates being enabled. By default all data rates are enabled and in order to shrink the cell/coverage, you must disable the lower data rates. Give that a try, most will leave 12 and up but again, depending on your environment, you can go as high as 24mbps as supported or required. You can reference this guide for autonomous.
https://www.cisco.com/c/en/us/td/docs/wireless/access_point/15-3-3/configuration/guide/cg15-3-3/cg15-3-3-chap6-radio.html#58958
-Scott
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