Cities Skylines - Can't select 3840 x 1600 in-game

[mva5580]

Hello, so I have the Dell U3818DW monitor and while it's fantastic, Cities Skylines seems to be giving me some trouble.

I had a Dell U3415W before this and was able to play in 3440 x 1440 no problem, but with this monitor whenever I go in-game in Skylines the only 21:9 resolution that it allows me to pick is 2560 x 1080. Nothing else shows up in that drop down. I've tried using the command line parameters to force 3840 x 1600, no luck.

Any suggestions and/or are there people here with the U3818DW who can select that resolution in-game? Thanks.