Hi There

I'm trying to recess a shower mixer valve on an internal wall of
perforated bricks. Theres a fixing bracket that needs to be recessed
about 5cm inside the wall and there is 2 holes + raw plug fixings for
this.

The problem is that the internal wall is made of perforated bricks and
so I cant get a good fixing.

Im guessing that I need to somehow fill the area behind with a filler/
resin type compound in order to screw into

Please help!!!