Resolution on Dell U4919DW with MacBook Pro 2017 two Thunderbolt

Hi - i just bought a very nice Dell U4919DW 49" up to 5120 x 1440 Pixels.

Before i bought it i had a look for the specs of my machine which are:

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"Video Support

Simultaneously supports full native resolution on the built-in display at millions of colors and:

• One display with 5120-by-2880 resolution at 60Hz at over a billion colors
• Up to two displays with 4096-by-2304 resolution at 60Hz at millions of colors
• Up to two displays with 3840-by-2160 resolution at 60Hz at over a billion colors

Thunderbolt 3 digital video output"

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But now the maximum to get resolution is 3840 x 1080 px

Ok - i ve got a machine with the Intel Iris Plus Graphics and i found in other posts,

that other machines with AMD Radeon do not have this problem.

But it can not questionned that if the official specs of the machine

are telling it IS doing 5120-by-2880 it should do it ( more than ever when its working with AMD )

Can anybody tell me how to handle that with the Apple support ? - I just bought both hardware

components because of this desired ( working ) set up. And now i feel really a bit kidded...

Thanks in advance for any advice