A property defined within a Piece is only visible to that piece.
A piece can only see properties defined within itself, or global properties defined in its current Zone, or Map, or the module itself.
Therefore, You need to create the BelgiumBRPCurrent property as a Global Property, either at the Map level, or at the Module level and then any counter can display the value of the text property via a Text Label displaying $BelgiumBRPCurrent$.
To change the value of BelgiumBRPCurrent using a counter, you then use the ‘Set Global Property’ trait. Or, you can define toolbar buttons and menus to do it by hanging change commands off the Global property.