LG UltraWide monitor not working with Xfinity Flex

I have an LG 34UM69G-B https://www.lg.com/us/monitors/lg-34UM69G-B-ultrawide-monitor

I'm getting nothing on the screen after the Welcome message from the Flex box (over HDMI.) Just blank where I think I should be seeing the screen where I select the language.

The monitor puts up a warning "The PC screen is not currently set to the recommended resolution - 2560x1080/60Hz. Currect resolution 1920x1080/60Hz. Please adjust the PC resolution."

But my computer can display on this monitor in 1920x1080 over HDMI. 

An Xfinity tech came out and checked out the Flex box, it's fine.  He was scratching his head over why the Flex box and monitor aren't playing nice.

Is Flex in 1920x1080? Anyone else using a similar monitor successfully? I'm bummed I can't get this to work!