I recently installed OperWRT on a Meraki MR18 and found the above guide very helpful ... thanks for that.
However, there were a few 'gocha's along the way I'd like to share, JiC annyone finds themselves in the same position I was.
- I tried a RasPi 2B and 3B+ and found the 2B to be most reliable option
- I used latest Lite version of raspios [ 2023-02-21-raspios-bullseye-armhf-lite.img ], which includes openocd Vs 12.
- I found it necessary to connect ground on MR18 JTAG [Pin 1] to RasPi ground [Pin 25] to get reliable results.
- I discovered a post saying the VCC should be connected if flashing a MR18 with 'newer' Meraki firmware installed. I connected pin 14 on the MR18 JTAG to pin 17 on the RasPi. That got openocd to load to the point where I could telnet to port 4444.
- I had problems the 'verify_image' step. I found a posting that recommended disconnecting all peripherals [display, KB, etc] from the RasPi to prevent this happening. It worked.
So, no major discoveries in and of themselves, but hopefully everything there in one place for anyone that needs it.
Hope that helps.