Thanks all for the input.  I have installed a DP to DP cable and that has resolved the issue.  To summarize, an HDMI to DP cable from the Dell U2720Q monitor to the AMD Radeon 355 card installed on a Dell 7070 resulted in no video despite multiple BIOS settings.  The cable did work connected to the onboard graphics, though, but at a reduced resolution.
I just received my Dell 7070 with add on video card installed--AMD Radeon RX 550 4 GB-- from Dell Refurbished.  I set up the computer plugging my monitor into the card's HDMI port, did not work.  So I plugged it into the mainboard HDMI port and was able to get up and running, albeit it at a low resolution (monitor is the Dell u2720).  It came with Windows 10, now have upgraded to Windows 11.  So my question is, how do I switch the video from mainboard to this card?  I'm reading that installation of the card automatically switches video to it, but that has not been the case.  I am not familiar with the procedure to get the card to work, please advise.
Activating an Add On Video Card