You need to build and flash the whole firmware with that kmod. The kmod dependency checksumming prevents installing kmods into builds that have kernel options that are not 100% same.
You could also try overriding the dependency check with opkg options, but that may cause the kernel to crash.
That is one of the reason, why I provide the easy to use scripts to replicate my builds, as people do have different needs, and may need slightly different kernel modules.
/And it makes no sense to build all kmods into the build. E.g. your wish kmod-ipt-filter might be useful if you would do string-based packet content sniffing in firewall, but I have no use for that myself.)