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I am afraid you misunderstood the concept. Using S11 on one side and S22 on the other side is wrong.

Think of your S2P block as a PI element, with series and shunt path.

If you convert to S1P using the method that I showed, you get the impedance between terminals 1 and 2 of the S2P block, with floating ground node. Using this 1-port  data is a valid approach as long as the shunt elements in the S2P are not (very) important. For example if your S2P is SMD component data, you can use this method. You then use the resulting S1P from circuit simulation in the CST model that you showed above.

So all you need is some circuit simulator where you can wire the S2P in the way that I showed above, with the circuit simulation port connected across the data block, between port 1 and 2 of the S2P block. Note that the circuit simulator block is NOT connected to S2P ground!

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That said, the more accurate method is to use an EM tool where you can properly define two ports to insert the S2P data. In your case, that would be one EM port from the left trace to common ground, and another EM port from the right trace to common ground. So you need a common ground in your model, and for accurate results it would be useful to have port calibration. CST does not support that, so for PCB work with SMD components a planar EM tool like ADS Momentum or Sonnet is the more accurate solution.