When will there be a clean Firmware for the X750?
Is there a way to create it by my own?
There is no clean firmware yet, you can download the openwrt source to compile it yourself.GitHub - gl-inet/openwrt: This repository is fork from openwrt official repo, we will update the latest tag and release GL.iNet firmware based on it.
You may like to try Rooter firmware as an alternative. 
Its “speciality” is 3G/4G modem support on OpenWRT routers.
I try to compile it by my own.