Lancelot allows you to turn on or off VPN clients on your ASUS router that are running Merlin firmware. As well as controlling your VPN clients from the app, you can also control them from a widget on your Home Screen or use Siri. You can also create shortcuts and turn on or off your VPN clients with automations or shortcuts in the Shortcuts app.
You can check if your ASUS router is supported here:
https://www.asuswrt-merlin.net