I am also using Redmi K30 5G (V12.0.6.0) and having the same issue, also the 5g network is totally not usable for now.
Without this option, it seems the 5g feature on this phone is disabled so that's why it's always connecting to 4g/LTE network all the time.
Before that, the 5g network is working well when using the stocked rom.
It would be appreciated if any update for this bug could be released at your easy convenience.