surround sound -- Multi Channel Stereo

We just "Upgraded" our Xfinity TV from cable to WiFi.  In our large screen room we have a Denon Surround Sound System which works well.  

With the xfinity using cable we could select "Surround - Multi-Channel Stereo" as an option.  Now wfuith the WiFi connection to the master box that option is not available, and we do not get full Surround Sound from back speakers and bass box.  Seems like only the center speaker works now.  Also with the old cable connection normal listening volume was around 40, now with WiFi and surround Sound set to "Dolby Digital +" our listening volume is 55 or higher.  

We use HDMI cabling from the Xi6T box to the Denon, and HDMI from the Denon to the TV.  Same cabling we used under the old xfinity box.  Also the Xi6T box is connected to the xfinity modem via a Ethernet cable.  

Multi Channel Stereo is available using the Blu-Ray Denon setting.

Not certain what to do make this work now.