It's not just the plaster that's cracked but the whole wall, hoping to fix it by just smearing a little in the crack is too much to hope for.
Ok, so what needs to be done? Maybe like this:
1. Dig out the soil
2. Use an angle grinder with a diamond disc and create an opening along the crack a few cm
3. Fill in with appropriate repair
After your quick fix, the newly repaired area or some new part will crack. Why is the wall cracking now? How does it look with gravel and ground fabric to reduce pressure and moisture, potential drainage? If I hadn't cared about the underlying causes, it would only have aesthetic value. Cutting up and trying to fillactror I'm not on.