Issue booting through TFTP on Meraki MR42

Hello, trying to install OpenWRT on a Meraki MR42. I get to step 4 on but unable to load .itb image using bootm:

Has anyone come across this before with u-boot, and if so, how did you fix it?

Managed to get it booted into the initramfs but there’s no mbn file in /root. I’m having difficulty working out how to transfer this file over serial as there are barely any tools available on the initramfs.