Dell Precision 7550 no microphone via 3.5mm jack headset

I use a 3.5mm 4contacts combo jack for my Sennheiser headset: works great on my E7470. The built in microphone switch is very handy. On my new DELL 7550, the microphone input is not recognized, so the headset is recognized as headphone only. Same issue on my USB-C dual connected docking station. Checked and updated all drivers: nothing helps: the microphone input on the PC itself stays default. Tried a BT-Sennheiser PXC550-II. There it worked: this device is recognized as headset. I think it's a Realtek driver issue, seeing both jacks on laptop and docking don't recognize the microphone on the combo 3.5mm jack.