@Yveaux Oh I see !!! I didn't know so many things can play a role.
Well, I am using a couple of breadboards onto which I mounted the microcontrollers and the radios. So maybe something isn't properly connected.
For the radios I use an NRF24 "adapter" that already has a voltage regulator and some capacitors onboard. So I did not add my own capacitors to be honest.
But I do power everything from my laptop's USB port, and the two devices (node and gateway) sit very close to each other. Actually one on each side of the laptop.
I'm gonna try powering each from a dedicated power supply, and I'll also try some space between them. I will also try different NRFs as I have a bunch. I bought 10 of them I think.
I will post my findings here. Thank you for your reply sir...!