Cannot add ifb device

Hi, I'm building a customized OpenWrt image for my project. I have enabled the kmod-ifb kernel module in the make menuconfig. But when I try to add an ifb device, I get this error:

# ip link add name br-lan-ifb type ifb
ip: RTNETLINK answers: Not supported

I am running OpenWrt in virtualbox.

Try modprobe ifb or else lsmod | grep ifb

I think ifb module is not loaded in the kernel

# modprobe ifb
failed to find a module named ifb

I'm sure I've enabled kmod-ifb, I can also see that it's already installed:

# opkg install kmod-ifb
Package kmod-ifb (5.10.176-1) installed in root is up to date.

I don't know why ifb module can't be found and not loaded.

I think the image builder is not compiling the ifb module. Here's a portion of my .config which clearly shows ifb was enabled along side igb and igc modules:

# CONFIG_PACKAGE_kmod-igbvf is not set

But only the ifb.ko is not present in the kernel modules directory /lib/modules/5.10.176. Both igb.ko and igc.ko is present.