Always keep one very important point in the mind while using softwares like CST or HFSS, you must be very clear what you are doing and you must have very clear idea what you are designing so that you could have a guess of frequency as these softwares define everything with reference to the solution frequency so if you make a very wired guess of frequency then all will be invalid starting from mesh to the solution. To guess about the frequency it is not that difficult as well as frequency is directly dependent on the size of components and you can have idea that what can be the solution frequency