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Compatibility of the WT-5 with the D810 through the UT-1 is listed; connect the WT-5A (or mount it upon) the UT-1, then when they are connected, you can connect them to the D810. And that should work, simply.
What symptoms do you get, after you connect them that way (the WT-5A connected to the UT-1 first, then the two as a unit connected to the camera) ?