Thank you for the reply.
I have another doubt. In the mode selection option of "mode expansion monitor", there are four options, i.e Fundamental mode, Fundamental TE mode, Fundamental TM mode, and user select. In the user select option, I can see only two modes are forming as the diameter of the antenna is less leading to a lower "V-number". Are these two modes corresponding to degenerate fundamental modes as there is structural symmetry in my antenna (cylindrical)?
I did the simulation by taking the contributions for all four options but getting different T_forward for each case. So to select the Fundamental mode should I go with the option of "Fundamental mode" or do I have to calculate the contribution of the first two degenerates modes forming in the "user select" option?

And for dielectric waveguide there is no fundamental TE or TM, so far I have understood the fundamental mode will be a hybrid mode as I can see in the simulation also the possible modes it is forming in the "user select" option all have almost the same TE/TM fraction(%).