'T - something and counting....'

[EddyCurrent]

So do you have these values entered ?

12-03 = 400
12-04 = 1

From the manual;
"The max preset line value of 12-03 is equal to the Maximum output frequency of the motor."

I don't know if you are controlling the speed via G-code S instructions but if you are the next challenge is to get that in sync. too. For example if I use instructions M3 S20000 it starts the spindle rotating at 20000 RPM and the vfd display also reads 20000 RPM +or- about 20 RPM. As you say a cheap tacho is almost a must to check the speed.