I have a slanted ceiling in bathroom curtin rod does not stay
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If you use a tension rod that stays in place by tension, you may need to switch to the standard rod which attaches to aflange on either end of the pole. They cost about $15 with the pole and I attach a photo. Here's how to install it.
https://www.hunker.com/13416411/how-to-install-shower-rods
If there are no 2x4's behind the spots where you want to mount the rod, then you will have to purchase special toggle-bolt screws that anchor into a wall with no support behind the drywall.
If you don't use these, the chances are high that the rod will pull out taking bits of the drywall with it, making holes, and it will be a mess to try to fix.
I had the same problem and used cup hooks along the wall at the same angle as the ceiling rather than a curtain rod. Then used those clip on curtain hangers.