Installing on a routerboard AP with just one ethernet port (cAP 2n)

I'm trying to install OpenWrt on a routerboard cAP 2n (using the common procedures) . I can upload the kernel+initramfs.elf, but can't contact it on
I suspect this is due to the fact that the only ethernet port is the "WAN" port and the initramfs is not configuring it as an input port.
Is there any config I can change and rebuild to try to fix it?