The issue is that chaos_calmer default image doesn't have enough storage to hold kmod-fs-ext4 on the MR3040. ug!
This is a good start, https://wiki.openwrt.org/doc/howto/extroot
but there's some other tips here.
So, you've got to use image generator to make a custom image that initially doesn't have luci initially, but does have kmod-fs-ext4.
This is much simpler if you've already configured the router and it's connected to the internet.