OpenWrt, OpenVPN and DNS Config

Hello -- I'm new to OpenWRT (recently converted from DD-WRT) and hoping someone can point me in the right direction.

My Config: Netgear R7800. OpenWRT v19.07.2. OpenVPN and VPN Bypass installed. ExpressVPN is VPN provider. I mainly use LuCI for config, but generally comfortable at the command line.

I have setup split tunneling using VPN Bypass to allow specific devices via ip to bypass the vpn. My goal is to have devices using the VPN to get DNS server from the VPN provider, and for devices bypassing the VPN to use either a custom DNS server or one set by my ISP.

The issue I am seeing is that even when a device bypasses the vpn it is picking up the DNS server set by the VPN provider. As expected, when I stop the only running VPN instance all of the attached devices pickup the DNS server set by my ISP.

I saw a related post that suggested going to Network->Interfaces->WAN->Advanced and unchecking Use DNS servers advertised by peers and entering an alternate ip address for dns server. After making this change and restarting OpenVPN all devices (both those using the vpn and those bypassing it) were once again picking up ExpressVPN DNS servers so that doesn't seem to be the solution.

Any guidance is appreciated.

The DNS configured in the Network-Interfaces are the upstream nameservers which dnsmasq will use.
By default dhcp server is advertising OpenWrt as nameserver and gateway.
So for the hosts that you want to use the ISP, this works fine without any changes.
For the hosts that will use VPN you need to assign them different nameserver. Here is an example how to do it with tags. You need to assign option 6 only.

If you have setup split vpn. You might need to tell the DNS forwarder to use the vpn gateway so that you don't DNS leak to your ISP.

Sent pm.