That is a great clarification on the use of #define MY_PASSIVE_NODE! Knowing that it works by simply letting the node skip all the 'normal' stuff (which wasn't needed anyway) and that there are no major disadvantages for this specific use case is super useful.

I’ve had similar trouble with the LGT RF Nano — the radio part is a bit picky. What fixed it for me was lowering the data rate in the sketch and giving it a proper capacitor on VCC (even a small 4.7µF helped a lot).
Also worth trying a different CE/CSN pin setup; these clones don’t always match the “standard” wiring perfectly. After that mine started talking to the gateway without any drama.