But, how? I just can´t get the lower part off, no matter what I do.
Any good tips on how to do it?
Yes...
Wrap Duck Tape tightly round the lower bit, get a pair of grips and simply do it. The tape should stop the teeth on the grips damaging the collar.
But you may mark the collar...I did. But the marks can be polished out.
Eventually.
Good luck....!