Are you sure the monitor is being detected correctly by Windows? If you open the Device Manager, it should be listed by its correct name under "Monitors". If Windows doesn't fully recognize it, it may be listed as "Generic PnP Monitor." If that's the case, try running the Dell installer that came with it, if you haven't already. Try installing the Dell Display Manager software and let it auto-update, then restart the computer and uninstall the Dell Display Manger if you don't want the software. That sometimes fixes Dell monitor detection issues.

For reference, I'm using a Dell U3417W, which is just the newer version. 3440x1440 resolution showed up right away in my game options after I installed the monitor; I've been running that resolution ever since. I use Windows 10 -- not sure if that makes a difference. Microsoft is notorious for lagging behind on supporting newer technologies on older operating systems -- like Win7, for instance.