Moving Personal Router to DMZ

Hi,

I need to be able to portforward on my personal router.  To do this I need to put my router in the DMZ of the Xfinity Router.   This should be an easy process.   I go to:

Advanced Settings > Connectivity > Expose personal router to internet

It finds my D-Link DIR-625 router by MAC address.    When I click forward I get:

"Router configuration failed” and there is a red X.

I've tried this over and over...anyone have a suggestion how to get my router into the DMZ?

Thanks,