Reddit reviews Laticrete 9255-0401-2 Ban Mini Unit - 1 Gallon Pail

This, otherwise you risk additional water damage down the road. The waterproofing membrane should also be covered up with a thick later of Hydro Ban or some other waterproofing agent. This is what I used when I did my DIY shower.

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Hydroban (you may find it cheaper at your local tiling store - and/or be able to buy "cuts" of the fabric for only as much as you're actually needing) is a paint on membrane that has some insane expansion capabilities (iirc around 9:1). I've used hydroban effectively as a crack isolation membrane before (it also has other uses) - it's probably one of if not the best product on the market at its price point. We don't really typically use crack iso a lot in NZ (some tilers have never touched the products with such intentions), so again, take that with a grain of salt.

Kerdi is a sheet membrane. I'm not familiar with it's expansion characteristics.

Applied directly to the concrete - under the SLC.

One other thing you could consider doing is chasing the crack with an angle grinder and masonry blade, then filling with a low viscosity polyurethane adhesive. But that's probably going so far above and beyond what you're needing to achieve.