RouterOS version?
For 6.40.2:
/caps-man rates
add name=default-rate basic=6Mbps supported=6Mbps,9Mbps,12Mbps,18Mbps,24Mbps,36Mbps,48Mbps,54Mbps ht-basic-mcs=mcs-0,mcs-1,mcs-2,mcs-3,mcs-4,mcs-5,mcs-6,mcs-7 ht-supported-mcs=mcs-0,mcs-1,mcs-2,mcs-3,mcs-4,mcs-5,mcs-6,mcs-7,mcs-8,mcs-9,mcs-10,mcs-11,mcs-12,mcs-13,mcs-14,mcs-15,mcs-16,mcs-17,mcs-18,mcs-19,mcs-20,mcs-21,mcs-22,mcs-23 vht-basic-mcs=mcs0-7 vht-supported-mcs=mcs0-9,mcs0-9,mcs0-9
/caps-man configuration
add name=default-configuration rates=default-rate
Precedences (6.40.2):
1) Defaults if nothing is specified on CAP Interface and wireless configuration selected on CAP Interface
2) directly set rates on CAP Interface
3) rates profile selected on CAP Interface
4) wireless configuration selected on CAP Interface
5) directly set rates inside wireless configuration selected on CAP Interface
6) rates profile selected inside wireless configuration selected on CAP Interface