To set the BSS Min rate on Fortigate controlled APs, lets say to min of 12, you can do the following on both 2.4/5:
config wireless-controller vap
edit Test-SSID
set rates-11a 12-basic 18 24 36 48 54
set rates-11bg 12-basic 18 24 36 48 54
end
These are the related settings to 2.4
1 1 Mbps supported rate.
1-basic 1 Mbps BSS basic rate.
2 2 Mbps supported rate.
2-basic 2 Mbps BSS basic rate.
5.5 5.5 Mbps supported rate.
5.5-basic 5.5 Mbps BSS basic rate.
11 11 Mbps supported rate.
11-basic 11 Mbps BSS basic rate.
6 6 Mbps supported rate.
6-basic 6 Mbps BSS basic rate.
9 9 Mbps supported rate.
9-basic 9 Mbps BSS basic rate.
12 12 Mbps supported rate.
12-basic 12 Mbps BSS basic rate.
18 18 Mbps supported rate.
18-basic 18 Mbps BSS basic rate.
24 24 Mbps supported rate.
24-basic 24 Mbps BSS basic rate.
36 36 Mbps supported rate.
36-basic 36 Mbps BSS basic rate.
48 48 Mbps supported rate.
48-basic 48 Mbps BSS basic rate.
54 54 Mbps supported rate.
54-basic 54 Mbps BSS basic rate.
and for 5 gig:
6 6 Mbps supported rate.
6-basic 6 Mbps BSS basic rate.
9 9 Mbps supported rate.
9-basic 9 Mbps BSS basic rate.
12 12 Mbps supported rate.
12-basic 12 Mbps BSS basic rate.
18 18 Mbps supported rate.
18-basic 18 Mbps BSS basic rate.
24 24 Mbps supported rate.
24-basic 24 Mbps BSS basic rate.
36 36 Mbps supported rate.
36-basic 36 Mbps BSS basic rate.
48 48 Mbps supported rate.
48-basic 48 Mbps BSS basic rate.
54 54 Mbps supported rate.
54-basic 54 Mbps BSS basic rate.