Letterboxing issues

I am trying to make a game, I made everything and it was working fine, but as soon as I tested it on another computer with a different resolution I noticed that my UIs wouldn't work, so I tried using letterboxing, I don't want different resolutions to see more than others, but this causes some of my UIs to go on top of the black bars and are therefore not useful.

I could solve this by making the canvases apply to a camera, but then my shaders get applied to the UIs as well and I don't want that.

I have also tried using anchors to make the UIs appear at a distance from the center, but I can't really seem to get that working.

I tried to use 2 cameras, one for UI and one for the rest of the game, but then I have to choose between my UI applying to the resolution of the player, or apply the letterboxing script to that camera, which places it in the correct spot, but covers the rest of the game in a black bar. Basically I need some way to either make a shader not apply to a UI that is rendering on camera space, which as far as I've gathered from Google can't be done, a way for my UI rendering on overlay space to notice the letterboxing and adjust itself accordingly, or a way to set a specific size for my camera and then have that scale up to match either the width or height.