Currently, all TGTS brushes for Affinity are in raster (pixel) format and can only be installed via 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.