3D foam aquarium background?

We just purchased a foam aquarium background in 3D and were told to use the Gold label Underwater Sealer. We found this was rubbish and not not work. Does anyone have these in their tanks and used it to paste?

I can use Geocel Sealants, it's like cement! Remember that these funds should be installed while the tank is dry, otherwise the water simply causes the bottom to the aquarium glass before the sealant has a chance to dry and cure properly.

Tanganyika Aquarium – DIY Underwater Cavern