For the ATTALK kit, concerning th emessage 'Failed to send packet'. I advise you to disable the ACK. We have seen similar issues where the Proximus LoRa network does not return an ACK, or was not received back by the modem, but the message did arrive. I guess you are experiencing he same issue here.
For your other issue. If you gat a valid DEVADDR, APPSKEY and NWSKEY, it should work. If you can not sort it out, i am happy to deliver you a set of working keys. You can contact me via the AllThingsTalk support: email@example.com.