@Sergio-Rius a Nrf24 radio can only buffer 3 incoming messages. If you send more messages at a very fast rate you might overflow the buffer at the gateway and start to loose messages. It all depends on the rate at which your node sends messages (that's why the delay improves things) and the speed at which the gateway processes incoming messages. I have some code under test which increases the buffer at the gateway side, by retrieving new messages as soon as they come in, which might help. Until then, just use a delay.

@martinhjelmare Wow thank you so much. I've cleared the eeprom and now it gets node id 1. After removing the persistence file the sensor now shows on the web interface.