The eco prime grip is supposed to go onto the top of the wundafloor boards prior to tiling.
You can stick the underside of the wundaboards down to the levelling compound with any c2 adhesive.
Are you a tiler or a DIYer, if you do not mind me asking as Wundafloor will only guarntee the floor using their tested method which is using Mapei adhesives and primers.
I have just tiled one before Xmas and it was a total **** up with the people who laid it.
One more point - you will need a crack mat on top of the wunda boards too as it is Travertine that you are fixing.
Good job I know what I am doing!!!