How to get DNSMasq to use remote (OpenVPN) DNS on DD-WRT router?

I have a DD-WRT router with an OpenVPN client. I'm trying to configure the local DNS server on the router to use the DNS server pushed by the OpenVPN server, via the OpenVPN tuel. Here's what I've got so far:

• Setup > Basic Setup
  • DHCP Server: Enable
  • Use DNSMasq for DHCP: Checked
  • Use DNSMasq for DNS: Checked
• Services > Services
  • DNSMasq: Enable
  • Additional DNSMasq options has interface=tun1 and no-dhcp-interface=tun1.

However, when I test which DNS my router is using, it's always the ISP's DNS server instead of the VPN's DNS server.