Underlay for saddle roof? Does side matter?
If I understand your question correctly, you have attempted to install a roofing underlayment sheet material from the inside/underside of the wood roof membrane in an attempt to prevent rain from getting in the attic.
That type of sheet material is designed to be applied to the exterior side of the wood roof membrane, under whatever final roof treatment (shingle, mineral asphalt sheet, edpm, etc.) is installed. See the picture below, which is from the website for the product in the link you supplied.
What you are trying to do will not work and will likely just redirect any water that does get through to a different undesirable area.