Distance for the Node to connect to the gateway MQTT-SN ?

Hi all,

I have a question.

When I try send message coap, it can send as mesh network.

But when I send search gateway msg, connect msg from publish (subscribe) Node to NCP. Seem it can't send as mesh network. I have to put Node in the coverage area of ​​NCP then it can discovery and connect to Gateway. If I put NCP --> Node 1 --> Node 2 in a straight line. Node 1 can connect to mqtt gateway, but node 2 can't . But I can ping from Node 2 to Node 1 and NCP. 

If I connect Node1 and Node 2 to MQTT gateway and put NCP --> Node 1 --> Node 2 in a straight line. I can send data from Node 1 and Node 2 to NCP as mesh network. 

I check message type of discovery and connect message. I saw it same with coap message. They are all used otUdpSend.

So Why is there such a difference?