sorry for the late reply, I didn't have the gateway with me to make a new trial. Anyway, this morning I couldn't get accepted on TTN using OTAA activation procedure. However, I would like to share few considerations about the results that I have got so far running both the poly_pkt_fwd and the tcpdump command.
"tcpdump -i ppp0 port 1700 -X"
The join_request is correctly sent to the webserver, and consequently the join_accept message is received from the gateway, so we can definitely exclude missing communication between gateway and server. Furthermore, looking at the first picture, you can also see that the RF packets is scheduled to be sent on a specific timestamp (still not sure who is defining it). I guess the problem stands in the last part of the communication chain, so the Arduino in somehow is not receiving the join_accept sent on Lora side from the gateway (for windows slots synchronization?)
To further confirm this result, I have also noted that using the other activation procedure (ABP) I can correctly sent packets from my Arduino node to the TTN backend server (they are shown in the TTN console), but I can't get back message the other way around as seen with OTAA (I simply tried to send few packets from TTN device view within the console but I couldn't see the packets back in the serial monitor, even though the downlink packets was successfully sent to the gateway).