VPN tunnel issues when captive network shares IP with remote VPN

AR750S, firmware version 3.215

Connecting to a captive portal via WiFi. Captive network is
I connect an OpenVPN Client to a remote The VPN works.
Beyond the remote VPN Server is another network that shares IP space with the captive portal network.
In other words, for example I have a DNS server several hops away. But because my captive portal network is also 10.0.., the router does not route traffic via the VPN tunnel. How can I fix that? In other words, my LAN clients connected to AR750S do not care about the captive portal network. I want them to funnel through the VPN tunnel to the 10.0.. network that is a few hops away on the other side of the remote VPN server.