If you have 42 breakers chances are your main supply is 3-phase. Look at this as three seperate power supplies coming from your energy supplier.
In that case, about a third of your house is wired to the breakers on phase 1, a third to the breakers on phase 2 and the last third to the breakers on phase 3.
Powerlines will only work properly on outlets which are connected to breakers which are on the same phase.
If your power outlet is connected to a breaker on another phase, there actually is no connection to the other outlet as another phase = another power supply! Hence no, or at best a very poor connection.
This is the most likely reason TP-link support told you chances of getting it to work properly are slim to none.
To find out which outlets in your house are connected to which breaker, and then to know on which phase that breaker sits requires some electrical plans of the house along with somebody (electrician) who can interpret them, or in case of abscence of said plans, an electrician who will need to do some detective work figuring it all out (time consuming but possible).
Then you could know between which outlets connections are possible!