Hi to forum

Recently I bought one JCUT 6090B CNC router for pcb applications.
Controlling software is ncstudio 5.4.53 with a dsp card . Being an
electronics enthusiast and after measuring with a tachometer speeds.
I found out I can only set 3 speeds and this is because the control to VFD
is based on 3 digital active low lines .I would expect 7 speeds though (2^3-1).
NcStudio shows LOW -MID -HIGH setting .That's all.I can't get any intermediate
spindle speed

I would expect a D/A for such machines to obtain more fine control of speed .
Anyway even using 7 steps is ok but not 3 . Anybody here knows what i'm talking
about and what solutions are there to correct the issue? .I'm familiar with electronics
but not VFD or CNC generally.
I'm kind of stuck here and the support I get is Chinese type .....