Installing the module does not automatically offload rules. You must explicitly tell nftables which flows to offload. This is done using the flow offload keyword in your nftables syntax.
: Ensure you are using a modern version of OpenWrt (19.01+ or current 23.05/25.12 builds ) as older kernels (prior to 4.14) do not support flow offloading. kmod-nft-offload
In simple terms: