Asus RT-AC86U runs slow behind Pace 5268AC in DMZ+ mode

Does anyone know why anything connected to my Asus RT-AC86U behind a Pace 5268AC can't break 40 Mbps on a speed test when the Asus router is in DMZ+ mode?

I currently have the Asus router NOT in DMZ+ mode and I'm getting a 192.168.1.x IP as the router's WLAN IP.  With this configuration I can achieve  650+ Mbps over Ethernet through the Asus and 200 Mbps on my phone over the Asus' wifi.

Is there any reason I need to use DMZ+?  Any benefit I'm missing?