Pi as DHCP Server and VPN Gateway

I currently have a Huawei Router which I use with a Tenda Mesh to server broadband around the house. The dhcp on the Tenda is truly apaulling, with to many faults to list here. This week I finally decided I'd had enough of it, and dug out an old Pi3 to use a DHCP server, at which point I had the bright idead that I could install Openwrt and also setup a VPN at the same time. So here is what I am trying to achieve.

The Mesh Wifi, is now acting in Bridge Mode the the Router.
I want the Pi to act as a DHCP server on the network. Using the Huawei Router as the main Gateway.
Then I want to have an IP address (possibly the address of the OpenWRT Pi) which when used as a gateway routes traffic through my VPN.
Ideally OpenWRT would recognise the devices at dhcp time, and server them the VPN gateway, however it's a shortlist of a handful of devices so if required I can set them up manually.

So I setup the Pi, with a single lan on ethernet, not using wireless (not a bridge, just a lan), setup the DHCP and that appears to be working fine, all good. So then I installed OpenVPN-openssh and the luci gui app. and setup my ExpressVPN details. However as soon as the activate the VPN (even without any firewall rules or routing), everything on the net appears to lose internet, even though it shouldn't even be routed through openwrt, it is using the Huawei as the gateway.

I'm guessing that something is wrong with my logic, or plan. Has anybody got any advise on how to achieve the end goal either with my plan, or a working plan?

Thanks

Draw a scheme of your network with devices, their roles, network interfaces, IP addresses, masks, etc.
This should help to localize, diagnose and understand the cause of the issue.

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