Openwrt firmware for Linksys EA9200
I'm not a professional programmer, so google would help you much more than me.
If you're on firmware 1.1.5 or lower, you can flash from Linksys UI. But if not, you have to flash with USB serial (refer below link)
Please remind that flashing this firmware can make your hardware unusable. I'm not responsible for any damage/issue this firmware may cause.
Please peruse the below link before flashing this firmware to your device. It's the guide for EA9500, but the basic is same.
https://serialize.wordpress.com/2017/02/01/linksys-ea9500-stuff/