This question is a little bit difficult to answer with the limited information available, but I will do my best. I will simply assume some things that seem reasonable.
Sodium intake does not have a direct effect on fat loss. But it still can affect your WEIGHT. But not the weight you might be interested in. :) In short, sodium affects your water balance so if you simply make modifications in your sodium intake any weight change is due to water loss/gain. This is clearly not the kind of weight you want to lose, right? :) A low sodium diet is still healthy though, for many different reasons.
Now, protein is a macronutrient (the others are carbohydrates and fats), so it does provide calories to the body and ultimately has an effect on the kind of weight loss you are interested in.
Here is the tricky part, it is hard to tell if you in particular will gain back the weight without knowing more about the diet you were on and what your diet looks like once you quit the low sodium/high protein diet. In general, if you do a short-term (even if it is months or years) diet to lose weight and then return to your old diet you will gain the weight back. This is simply common sense, since you had to gain all that weight in the first place with your regular diet... But you probably already know that.
So, maybe what you mean is: what happens if you made general modifications to your old diet already and then did a special diet for a shorter time where you increased your protein intake and lowered your sodium intake... Will you gain the weight that you lost on this short term diet back even if you simply return to your already healthier diet. If you lost a significant amount of weight during that short-term diet compared to the other dietary changes then the question is most likely yes. I assume that if you increase your protein intake you decreased some intake of another macronutrient to keep the calorie balance, this macronutrient was most likey carbs. And a big part of the weight that gets lost on low-carb diets is water. If you decrease your carb intake enough your body will take the energy it needs from "carbs in storage" (aka glycogen) which is stored together with water, which then also gets released, resulting in weight loss, water weight loss. :) Your body obviously wants to get this stores back as soon as possible since they are important: the brain for example works mostly on "carb-energy" so if you do not eat enough it just takes it from the storage reserves. So as soon as you eat more carbs again it puts some back in storage with water and you gain weight again.
In general, any weight that you lose on a diet will be gained back unless you make long-term changes to your diet.
I hope this helps. Let me know if you have anymore questions or want to clarify something.
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