https://openwrt.org/docs/guide-user/luci/luci.secure
fwiw, refer also to section 10.4 of the installation guide for HH5A PDF linked below if you are a Windows User based on above openwrt wiki page. It describes 3 methods for accessing openwrt router through the WAN. See also section 9.16 for DDNS if you do not have a static public IP address.
https://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=266
If you are using an ADSL 'router', you will also have to set up port forwarding rules on this 'router'.
Perhaps you could use LuCI to stop and start the VPN instance? You will need to remove the 'kill switch' otherwise devices won't be able to access the internet when the VPN instance is stopped - see section 2.4 and 2.8 of the openvpn client guide you already for ideas.
Alternatively, look at VPN Bypass option, or VPN Policy based routing
https://forum.openwrt.org/t/vpn-policy-based-routing-web-ui-discussion/10389