I purchased a couple of MR52 listed as "unclaimed" on ebay. When I powered them off a POE switch, they booted and obtained an IP address from my Openwrt router and gave me a login page through SSID "Meraki Setup" it created.
I am trying to flash them with OpenWrt.
Does "Meraki Setup" mean they have been already set to defaults?
Does Meraki MR52 only offer pathway to OpenWrt through the serial interface?
My educated guess (I've never actually used any of the Meraki kit): One of three possible scenarios: reset, never even setup, or license revoked/expired. They require a licensed activation (with the vendor firmware, doesn't apply to OpenWrt), so you need to go through that setup to use them in that context.