no ultrawide thru the USB-C display port?

so I have a Dell Ultrawide monitor, and when I was running HDMI to HDMI, everything worked fine.  Could do the ultrawide screen with no problem.  I figured the DP must be better, at least capable of higher resolutions, so I bought a USB-C to DP cord (my laptop doesn't have a straight DP to DP port).  It technically works....but I can't figure out any setting to change to make it actually fill the screen.  Not to mention the color depth looks like it is locked at 8 bit, don't know if this is normal.  Gonna go back to the HDMI cable if I can't get it to work, but since I bought the "nicer" stuff, it would be nice if it worked like it was supposed to.  Help?