After solving the crystal issue, I just connected a KNX power supply to the board and measured on the TXD pin of the NCN5130 using my logic analyzer.
Seems like it is working:
After startup, the transceiver sends a U_Reset.Ind signal (0x03 hex) to the micrcontroller to indicate everything is ok and it has booted. This means that the HAT is up and running!!!
Now the next step is to plug it onto a Raspberry Pi and to get it communicating with KNXD.