Thank you both for your answers.
With so many parameters to play with, it was not easy to understand what was wrong.. But I think I finally got a way do get my hardware working
Here are my findings :
I thought I got RFM73 version of NRF24-like radios. They look like like here and here.
After checking and connecting all VCC and GND together, switch radios and arduinos... I got them suddenly working with Mysensors development librairies, then back to silence without apparent reason...
Indeed like @mark_venn suggested, I went back to stable librairies and try so set a static ID. Worked well... until it went back to silence again !
Then, following @mfalkvidd advice, I realised that flashing the eeprom_clear sketch solved my all my problems !
My summary :
Ensure that your 3.3V power supply is strong enough.
Ensure that your arduino, radios and sensors are sharing the same GND and VCC.
Use development librairies from github
Flash eeprom_clear before flashing your mysensor sketches
Then gatewaySerial and relayActuator should be able to communicate "out of box", mean you'll monitor some lines like this on the gateway serial : 0;255;3;0;9;read: 0-0-0 s=255,c=3,t=15,pt=0,l=2,sg=0:
Thanks again for your help, now I can start to play
I see a lot of "!TSF" and "NACK" which seems to be radio communication problems.
That's MySensors related and can be related to many things, from soldering problem to power / capacitor issues.
In MY_DEBUG mode you should see your temp/hum printed on serial every ~15s so there is something wrong there, either wiring or a code issue.
Ty with the example included with the library "i2c_SI7021.h", without any MySensors related stuff, to see if your sensor is detected. You should see temp/hum on the serial.
Thanks for the tip!. It looks like a USB powered Arduino Nano cannot handle the long range radios power requirements. I powered the radio module independently and I can send commands succesfully again!
The range has not improved, though, but that's another matter...