After some difficulties described here I'got OpenWrt successfull running on my first Meraki MR 18 AccessPoint.
Even the firmware on that device was 25-201712141532-G0bb86dd3-hamaki which is flagged on the device page as "Not posible to install OpenWrt".
The second device I have has obiviously a different or newer firmware.
On the UART console the odm command seems to be not available anymore odm help brings you a "UNRECOGNIZED COMMAND LOGGED TO CLOUD SERVERS" back.
Any idea how to flash this device - JTAG? Anybody has some experiences?