28 MHz is a pretty small frequency to mix with, and the mix products should be attenuated properly. In the mixer there is some filtering, but the documentation advices to add some more dB's attenuation. So, I build a filter from PCB board and three resonance tubes. The design I used is from

here

And the result is this.

I used a nanoVNA to measure the filter and to adjust it to the 2.4GHz. The filter is 15MHz broad and has a very good characteristic. Look for the picture.