@ReneB. 

Normally, the Deco system will recommend the client device to roam to the closest Deco unit when the device is moving around. However, whether to roam or not depends on the device itself.
Some devices may stay connected to the original Deco unit rather than connect to the closest Deco, possible reasons include the following:
1) The device doesn't support the roaming protocol. Then it won't roam until it disconnects from the Deco unit.
2) The device may regard the signal strength of the current connection is still strong enough and don't want to roam to another Deco unit. It's decided by the roaming threshold which is set by the vendor of the device.

You can refer to the link below to change the settings on your client to have a try.
https://www.tp-link.com/support/faq/592/