Collecting NMEA data on uart1

Hello, I am trying to configure the F9P rover to output NMEA data on UART1.
I would like either to use a 433MHz transmitter to beam the NMEA data to a remote PC with a paired 433M module or to use a datalogger to record the data. However, I am unsuccessful in getting data out at uart1.

My setup is:

The F9P rover has been configured using standard rover configuration and uart1 has been enabled with the port configuration with protocole in/out 1 - NMEA.

The F9P base is not used in the first trial.
The rover is up and running with the USB port connected to a PC with u-center 19.04: the data are present and the fix mode tells 3D.

No device is connected to uart1: I probed with an oscilloscope the Tx pin, no signal is present.
With a 433M module connected to the uart1 port, the result is still similar, no signal as well on tx pin.

I then turned on a F9P base transmitting correction data on a 433MHz, the Rover has same SW configuration and a 433MHz connected on the uart2.
The fix mode is 3D/DGNSS/FLOAT.
But still no data on the uart1.

Is there a special thing to do to get data on uart1?