Welcome to the forum!
To get the best out of the XR500 and DumaOS, it's be best to replace the OpenWRT router with it. The Wi-Fi on the XR500 is pretty good, better than your standard ISP router anyway!
With the XR500 here in the network, you won't have any issues with NAT and QoS will have fully control over the network, ensuring you're not having any issues with lag due to a congested network.
All DumaOS work regardless of whether you're using DHCP on the XR too!