TTN doesn't have downlink support yet as far as I am aware although I think they are working on it.
To use semtech server you will have to register your device at their website where you can set the AppSKey , NetSKey for a device devEUI or use OTTA configurations. I have tried both ways and it does work.
therefor I do not know how to parse the data...
How do you parse the data now? In all cases you will have to be able to use different keys than the default TTN test keys so you might aswell figure out how to do that with the semtech server as a first step.