The physic is the problem and there is no way around.
For higher modulation rates you must have higher SNR (more cleaner channel).
So in noisy environment it is actually better to use lower (manually set) rates/modulations
which are almost the same in both 802.11ac and 802.11n radios.
And because there is almost no manual control over 802.11ac rates and power settings in RouterOS
older manually fine-tuned 802.11n radios in most cases are working better in noisy environment.
Regards,
M.