In fact, I'm an i d i o t, as the laptop needs to be docked for the serial port to be populated in the PnP device, even if it's only wires from dock socket to DB9 plug AFAIK...
Well, as subject says, after updating to BIOS 1.26.3 (dated 2021 July IIRC) I lost my serial port in Windows, it does not appear anymore in the Device Manager, I only have the LPT1 port. It is still in BIOS, I tried to disable it then re-enable as COM1, but still not there in Windows 
E5470 lost serial port after BIOS update