How do I lock a wifi only device if only online occasionally

Hello!

I have a Sony Z3 tablet with no sim card- So obviously I cannot send sms commands.
I sent a lock command from the Avast web to the device while it had no internet access. When wifi access later was available nothing happened. The tablet did not lock. I had hoped that the device would have been locked as soon as the device got internet access.
Is this not supported or am I doing something wrong?
The work around is to send lock commands every now and then until you see that the device is locked. But that could take a considerable amount of timeā€¦

// Robert