Hi again, batchess !
Please mark me in the "challenge accepted" category.
Samsung does claim Mac compatibility with this display. It would appear you are limited by the MacBook Air itself. To wit, the Samsung U28E590 (I believe you are using the LU28E590DS/ZA) carries a native resolution of 3840 x 2160. Unfortunately, your MacBook Air (13-inch, Early 2014) at a maximum can only display 2560 by 1600 pixels on an external display. You are necessarily limited to a lower resolution on the Samsung by the hardware of the MacBook Air, and that lower resolution may not (read: probably won't) display correctly on the Samsung. I apologize for preaching to the choir on this point.
Resetting the SMC or NVRAM will do no good, and will not fix this issue - no amount of resetting the SMC or NVRAM will increase the resolution your MacBook Air is capable of displaying to a monitor and match the native resolution of the Samsung.
I apologize for the misinformation being sent your way, especially by those proposing a fix to a hardware issue without looking up the relevant specifications.
I wish I had better news for you, but bid you good luck !