Thank you @PhilipDAth @ww @DarrenOC for your nice suggestion. After conversation with Cisco Meraki Support teams, they ensure that, there is a problem with regularity domain. In our country (Bangladesh) UNII-1 channel is not support on firmware version 27.5. After rollback to 26.8.1 from 27.5, controller shows rollback successful but back end is stable firmware version 27.5. For the regularity domain set to Bangladesh, AP is rebooting constantly. Finally support team changed the regularity domain from Bangladesh to India in back end, now AP is stable. Now the controller shows firmware version 26.8.1 & support team also confirmed that back end also stable on 26.8.1. When firmware version is stable on 26.8.1 we communicate with Meraki support team. They changed the regularity domain to Bangladesh from India. But same problem again "AP is rebooting". Now temporary we use regularity domain India.
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