RBmAP2nD not booting from initramfs

I'm trying to install OpenWRT on a Mikrotik RBmAP2nD, which is supported as ar71xx target.

In line with the wiki, I'm using TinyPXE to boot the openwrt-19.07.8-ar71xx-mikrotik-rb-nor-flash-16M-initramfs-kernel.bin image. TinyPXE shows a DHCPd:ACK followed by a TFTPd DoReadFile indicating the initramfs image has been uploaded to the device.

However, after this, nothing seems to happen. I have set my ethernet to a static, trying to ping/ssh OpenWRT on but the device won't respond. Also when trying DHCP, the device won't give me a lease. I've tried both ports, and also tried older builds on the initramfs image (19.07.2 and 18.06.1). After a power cycle, the device will boot RouterOS again.

The device is running RouterOS v7. Could it be this has an updated RouterBOOT bootloader that somehow interferes with OpenWRT initramfs images? I can't seem to find a different reason for this device not booting...