Out of curiosity, why are you running a 1+ yr old firmware that is no longer supported? It would be more beneficial to you to upgrade to OpenWrt 18.06.1.
The dependency issue is likely due to buildbot compiling new packages over a year ago and one of the required dependencies required for kmod-nf-nathelper-extra is compiled against a different kernel version.
General rule of thumb:If you don't know precisely why you're receiving a dependency satisfaction error, forcing the installation of the package isn't a good idea since it can wreck havoc, and brick, a router when dealing with kernel mods.
Easiest, most efficient, and secure solution: upgrade to the most current firmware version, 18.06.1