As far as I can tell, you need to install the Simplygon SDK and then use their installer to make Simplygon available to UE, but they don’t provide that installer for UE 4.19 anymore. I don’t believe there is any way to get Simplygon running in 4.19 which is why I stay with 4.18.
It’s really funny how Epic and/or Simplygon are messing this up. You can’t very well use assets without having some sort of LOD reduction. there is Instalod but they stopped supporting indy developer and don’t even give a price anymore on their website, which probably means it’s astronomical.
The only hope is really to wait if Simplygon provides a way to integrate their product in UE 4.19 again. You can go to their site and ask but I haven’t read much positive info in their forums regarding that matter