You have unrealistic expectations for the current level of development of human science.
We can’t decide in which parts of our body the fat will be stored. The default is in the center of our body mass from where it can be easily and quickly distributed wherever it is needed.
The center means tummy, in case of women it’s more often buttocks.
So, if you don’t lose weight or rather lose fat, your tummy will not get flat. No way.
When you exercise and burn fat, it’s burned evenly from the stores of the whole body. No matter if you train legs or arms, the fat is used evenly from your whole body.
If you want to maintain your weigh and get a flat stomach, the only way is to burn fat (from the whole body) and build your muscles (the ones you decide to train).