For those who have added usb mod into your wr941nd, you can use the patches in this branch
This is the patch and commit to add usb support.
Remember you need to select these packages (in make menuconfig) for usb to work:
block-mount kmod-fs-ext4 kmod-usb-storage kmod-usb2 kmod-usb-uhci kmod-usb-ohci
Also note: WR941nd heats up in 20.02 while high network usage, so you need to add heat sink on cpu and marvell ethernet switch or some fan.(otherwise it will reboot after getting heated)