I am no expert, but I don't believe this has anything to do with ultra. I run 4 monitors and works fine, with some glitches in 1.5 which may have to do with my Track IR (exported screens/instruments fade/go in and out) and total failure of DCS 2 due to a "shader" error, but it works.

First- make sure your phone is added as a fourth monitor in Windows and works as a monitor
second- ensure your far left monitor your primary monitor.
Third- in DCS settings, set to single screen
Fourth- in DCS choose your screen resolution, My settings for 3 1920 x 1080 monitors plus one 1366x 768 usb monitor is set to 5760 x1080

You should now be able to see DCS stretched across your 3 primary monitors and then be able to drag the exported instruments to your 4th screen. Now, I would choose something larger than and I phone to export the instruments, My small monitor works great for export, I have 2 Cougar MFDs velcro'd to the monitor and the exported MFCDs are adjustable and fit inside the Cougar MFDs with the buttons matching the MFCD function buttons, there is also space to fit the CDU RWR. etc.

Good luck, hope this is helpful.

Berdman