U2717D, 2010 MacPro, Ubuntu Studio 19.10

I have a 2010 MacPro (2.4 GHz, 8 cores) with a Radeon 5770 video card. The card has two mDP and a DVI connector.  I am running Ubuntu Studio 19.10 on this computer. Note - the U2717D works fine with my Dell laptop running the exact same operating system, so I do not believe that I am having an OS issue, but a video card issue.

I plugged the U2717D into the computer after the computer was running. The U2717D was connected from the computer mDP to the U2717D DP using the cable supplied by Dell. The computer correctly identifies the U2717D with respect to both the resolution and the refresh rate. However, I get a message on the U2717D stating that the resolution is not correct. I saw in this thread that I should change from DP 1.2 to DP 1.1. I held the DP button down for 8 seconds, then disabled DP 1.2 (I did NOT get the option of setting it to DP 1.1).  However, I still had the same error message, incorrect resolution setting.

Do I have to reboot the U2717D and/or the computer after changing from DP1.2 to DP1.1? I noticed in the instructions (which I found online, since there were none with the U2717D) that I should plug the U2717D into the computer before turning the computer on. Is this critical? I did not think it was, since I never noticed an issue before with monitors.

Any and all suggestions welcome to allow me to use this U2717D.  Thanks.