Wall-mount Kimera on Geberit in-wall tank holder

I am putting a Porcher Kimera toilet on a Geberit in-wall tank and holder system. The problem I am having now is that the seal on the discharge line leaks. So far I have tried the grease that came with the installation kit, vaseline, bicycle bearing grease, and silicone sealer on the seal. I am not sure the silicone sealer will not work-- I am waiting for it to dry now. I thought, though, that I might be overlooking something. The seal on the holder system that receives the discharge pipe looks questionable to me-- as though I am missing a sealing ring. The rubber seal lip is cambered towards the central hole to lead the pipe in. Behind this lip is a cavity about 1/4" wide and about 3/8" larger in outer diameter than the discharge pipe. At the other side of this cavity is a molded pipe edge with the same internal diameter as the discharge pipe from the toilet that this dischage pipe's end should rest against. It all looks a little odd to me. This one flimsy seal lip is supposed to be water tight with, I guess, some added grease. It hasn't been close to water-tight yet. I am trying filling this cavity with silicon sealer this time. There was nothing said in the directions about doing so, but I hope it will work. Anyone have ideas or experience along this line they would care to share?