Most TGTS brushes for Affinity with the exception of Bézier Buddy, are in raster (pixel) format. This means that they can only be successfully installed in the Brushes panel when Pixel Persona is activated.
To install your raster brushes activate Pixel Persona, then open the brushes panel by going to View > Studio > Brushes. (see below).
This is obviously a little confusing because the Vector Brushes and Raster Brushes panels are both named "Brushes". The only way to tell the difference is by checking whether you are in Designer Persona or Pixel Persona.