The problem encountered by the Dell Monitor Driver Installer (DMDI), and by manual installation, indicates that the security catalog (u2721de.cat) has an invalid digital signature. Windows won't allow you to install it, manually or via the DMDI, because it cannot verify the publisher (Dell).

You can wait for Dell to post an update or disable driver verification temporarily to allow installation, but so long as Windows can read the monitor EDID and recognize it as a U2721 or as a Standard or Generic Plug and Play monitor (as it does in the screen shot in the other thread), you really do not need the driver. The EDID provides far more, and more accurate, information than the monitor "driver".

Monitor "drivers" just (a) provide information in an inf file about the monitor's recommended resolution to systems that cannot read the monitor EDID, and (b) include an icm color profile for use by color-critical software like Photoshop. If your monitor is identified correctly by name or as Plug and Play, you do not need (a); and if you need (b), you can manually install the ICC profile by right-clicking on the U2721DE.icm file and selecting "Install Profile" from the popup menu and then make sure your color critical apps are using it by running the Color Management Control Panel (colorcpl.exe).