Linksys EA9200 Installation?

on the toh EA9200 has squashfs.trx file but im unable to do anything with it. im completely new to this so i dont really know what to do. ive got a laptop connected to the router via ethernet and am trying to upload the custom firmware but its saying the firmware file wont work on the router. ive also downloaded the stock firmware thats in .img format but i dont know how that helps so anyways, does anyone know how to install openwrt on EA9200?