Gripfill is probably best, the solvented type in the green tube. Use plenty, but not so much that it squeezes from the edges when you push the upstand to the wall. If you can, wedge the upstand in place whilst it dries, in the worst cases you can wedge it with a piece of timber from the opposite wall.
Once dry you will need to stop the dried gripfill from becoming wet or the upstand will come away from the wall with the plaster. To stop this silicone the top and bottom edges.
If you are painting the walls, silicone after you have painted as the paint will not adhere to silicone.
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