XXL RTK Base is not recognized

When using the u-center v8.28 I can see that the XXL RTK connects to my PC via Sensor API 0 network connection. But QGroundControl only accepts connection via COM 4, COM 5 or COM 8.

How do I change XXL RTK to communicate via one of the serial ports ?

You can try this tutorial : USB Driver for Ublox NEO-M8N GPS + HMC5983 compass (XL)