The instructions on how to install Openwrt on Mikrotik routers is outdated.
For RouterOS versions newer than 6.46.6, the router expects the initramfs file to be called "vmlinux".
This means that for TFTP boot via dhcp/bootp to work, the initramfs.bin file must be renamed to plain "vmlinux".
I struggled for hours to install Openwrt on my hAP ac2 (and I had done it many times before), lucky I stumbled over some guide that mentioned this. (http://www.aturnofthenut.com/2021/03/17/using-dnsmasq-under-openwrt-as-a-tftp-boot-server/)
Hope to save someone a lot of time.