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.

Great tip. For anyone else struggling with this, I found that defining MY_TRANSPORT_WAIT_READY_MS to a very short duration (or using MY_PASSIVE_NODE) also helps keep the sketch running when the gateway is offline. It's so much easier to debug logic when the radio isn't a bottleneck.