There seems to be a bug in the building of grenzers, I found if I rebuilt the military governors building in the right area I could then recruit them,if I already had the right building in place before researching the tech to unlock them they wouldnt appear.

This is with a vanilla ETW install no mods etc

The important part is they can only be recruited in 2 areas if you read the description but I only recruited them in croatia, I think it is, not from trannsylvania so no idea if both areas are like this i've never seen them in the latter area.