CloudKit, avoid 2 devices to use simultaneously the private database using a subscription

I'm trying to do something quite simple. I want to avoid 2 devices with the same appleID to use the private database at the same time.

For this purpose I thought about using a subscription. This way whenever a device enters the game it updates a register with the deviceUID. And if another device (using the same appleID and private database) is playing, gets notification and I let him know before kicking him out of the game. (We dont want 2 devices to use the database at the same time)

Is this possible using the subscriptions? or did I misunderstood something? Because I was able to create the subscription and the code is supposedly listening to it but I dont get any notification by code when the register declared as 'suscriptionType' is modified or even deleted.

Thanks a lot.