Hi Jan,
Changing the Rpm DRO for the 2017 screenset to 2451 is needed for controlling rpm, technically it should still turn spindle on/off depending on what rpm VFD is set to, it actually should still work when you start UCCNC with VFD off than turn on. The only time it won't turn on spindle if UCCNC/2017 ss rpm is set lower than your minimum rpm settings in plugin. If I remember correctly you would at least hear the relay click inside VFD. Sorry to say I'm suspecting other issue's here other than wrong configuration set up.
Once I seen you were using and older plugin I looked no further assuming that was your problem. I'm just home from work so need a little time to review your profile and adapter and will get back to you, I'm sure will debug where your problem is.
BTW what version UCCNC are you using?
Dan