Docgi

@AmateurRadist If you look at the first picture you send "ground" is look unassigned so there is no reflecting plane in that simulation, assign PEC boundry. Also as you said your design file didn't have S11 plot so I put it myself when I simulate it. To lower VSWR I think you should focus on feeding line and patch sizes. Maybe you could cut feeding point of patches like in below picture. But I think feelind line is more important in your design

@Admin, Is there a CST file you could share ?

I tried a similar design to this and hell yeah looks like it works.

@AmateurRadist I simulated you design and then made some changes. First there are many mistakes I beleive you need to fix and I'll show you some. I did another simulation for wider bandwidth to see antenna performance. Looks like there is lots of optimization to do because if you look at the results antenna has minimum return loss at higher frequencies but max realized gain at lower frequencies. So the whole design should be optimized I guess.

This is also the warning you get, because you also choose solve material inside for conductive material. I don't recomend that option for conductive materials, unless you want to look the Skin effect. If you unclick it the warning is gone

@Admin @AmateurRadist There is ground plane in the design but I realized that you selected "Display Wiframe" option on material properties. If you unclick it you can see ground plane easily

I also tried to simulate this antenna. I had return loss shows that this antenna can work between 800-2700MHz with little tuning but I couldn't get gain up to 5dBi

It looks like there is a ground plane but you made it transparent. If it's possible, would you like to share the aedt file with me. I can examine the whole design for you and maybe optimize it.