You seem to misunderstand what a mutation means. A mutation is a random change in a gene. That happens millions of times in millions of cells in our body. Many of those mutations cause the cell to die, some don't change anything, some may make it get better at something, like not dying - putting it on the path to cancer.

If cell#74729194 mutates and becomes cancerous, it divides, divides and divides. If chemo poisons it and all its descendants, You should be cancer-free.

In reality, the more a cell's DNA is altered, the less stable it becomes, which makes every next generation of cancer different from the previous ones. That in turn makes it so all the cells are different, which can cause say 70% of the cells to respond to chemo while the rest won't.

The other problem You touched upon is what is considered a remission - yes, if we don't find a trace of it, we will consider it a remission - but we can't deconstruct a human to check every cell if there aren't 3 metastatic ones in that person's brain, one in the little left toe, and some next to where the tumor was. Best we can do is find a new small tumor, or some activity suggesting a metastasis, which can look similar to say an infection (and inflammation).

It is complicated, I think the first 2 paragraphs show what You wanted, and the rest is more corrections. Hopefully they are clear.