Any known firmware alternative for gl router hardware?

@Henry_Bruns

How to configure DNS, VPN and a Killswitch by Openwrt ? Does it need to install additional addons or how to do

I suggest you create 2 AP, one passthough, one vpn.
This will simplify the operation with captive port authenication/vpn killswitch/dns leak etc Look up policy based routing and guest (VPN) network configuration.