Single Channel LoRaWAN systematically accepts just one packet out of 3 sent by node

I just built and tested a single channel LoRaWAN gateway which is connected to TTN as per the instructions of thing4U/esp-1ch-gateway with a single channel node both based on TTGO-ESP32Lora and eventually configured both on www.thethingsnetwork.org. Everything works nicely but I do not understand why despite the node sending data at pace of 2 minutes, the gateway receives just one packet out of three. So if I trasnmit: packets 0,3,6,9 etc. the data at ttn is updated every 6 minutes instead of 2.

That is correct. LoRaWAN uses the first three channels as main channels for communication. More can be configured for use. These three exist in part because they then can always be used for OTAA.

So if you have a single channel gateway and it is listening to 868.100 MHz and your node sends on 868.300 MHz then your gateway won't hear it because it is listening on the wrong frequency.

There are several solutions:

• configure your node to only send on the single frequency your gateway is listening for.
• Add two more single channel gateways who listen on the other main frequencies.
• Add a multi channel gateway.

Frequencies are only meant as an example, these frequencies are applicable to EU and may differ in your own region but the principle still stands.