Hi @htdvisser
First of all thank so much for taking the time to reply
Got it!
I think it was database were outdates or so, I stopped the whole server and re-register the devices
Since I am running TTNBackend in a docker container, what I have done is pointing that variable to my docker IP address, as you told me. Now I am able to receive data from my motes! 
However I have some further questions,
I have noticed that after a while my user token expires,which means that my motes are deregistered from the network and they cannot send data, obsiously on the other hand, getting back some errors, since I am not connected to internet I have to reconnect, re-register my devices again.. Therefore, I was wondering, is there any configurable parameter to make those devices permanent? Kind of lease file or so where I can spicify it, I have seen the auth.json file that contains my token and the expiration time, but that is something generated by the account.thethingnetwork right?
Regarding to the downlink, I am using node-red to create a package (the same as the wiki says) to send to my mote, I can see in the terminal of the backend that the handler enque the request for downlink, but I don't see anything more than that. My device doesn't seem to receive anything, so I was wondering if there is some way to check that that package is reaching the gateway and being sent. Any hint?
Thanks again!