@Asyn: Unfortunately, this article does not help me.
MyAvast account shows me my mobile phone with a red dot. So it is not reachable although I have my GPS on and I also have signal. I cannot send a "Locate" command.
@Tereza V.: No, I don't use them. I already use the Anti-Theft function which is integrated in Mobile Security Pro. The Anti-Theft feature is also enabled in my mobile phone and the app is connected to my Avast account.
@All: Attached you will find a picture from the overview of my Avast account. The XT1580 is my mobile phone.