Hi,
I'm quite desperate about this issue - 
I have a main Deco unit in the office room and 2 other units - living room and bedroom. The living room unit is closer to the main office unit and the bedroom unit is closer to the living room unit.
At first setup the bedroom unit connected to the living room unit and I received great speeds. The problem is that each time the electricity goes off or the connection is lost or I try to do a network optimization the bedroom unit connects to the main office unit and I receive poor speeds in the bedroom.
When re install both the living room unit and afterwards the bedroom unit - the bedroom unit connects to the living room unit and I receive good speed again. But it's very frustrating to have to do it almost on a daily basis.
Any solution to this? I read in the forum and saw that currently there is no feature to allow connecting a unit to a manually selected other unit.