CST simulations at sub 1 GHz frequencies

I have been working with CST MWS for a few months, running simulations for testing 2.4 GHz antennas and their performance with various interferences.

Recently I tried to do similar testing for the 915 MHz band. However I seem to be getting either wildly inaccurate results or the solver crashes.

Are there settings that must be changed in order to get accurate results when working at less than 1 GHz. Any help would be appreciated.