I still have a tiny problem.
I used the Mycelium wallet. It use some kind of seed to produce a different private key for each transaction you do. Apparently, there no way to access the private key you use cor a particular transaction. The only thing I have when I export the wallet is a very long weird key as that one :
(Don't worry, it's not the one they gave me
It's more a problem with Mycelium than it is with Bts Play. I post this just in case you have heard someone having this issue or if you have some ideas of what I can do.