Once you have OpenVPN connected on your Merlin FlashRouter you can enable the VPN Killswitch so that no internet traffic passes through your router that is not encrypted by the VPN tunnel.
Login to Asus Merlin firmware settings in a browser on any computer or smart device.
1. Under Advanced Settings select VPN.
2. Click the VPN Client tab.
3. Set Killswitch – Block routed clients if tunnel goes down to Yes.
4. Click Apply.