Based on my experience, the attachable lenses I have tried only do one of two things:
1. Provide a wider view of the lens (may it be a wide angle are a super wide/fisheye view)
2. Provide a closer view of the lens (telephoto)
They do not, however, increase the megapixels because megapixels are internal to the camera inside your phone. I also notice that the quality of the picture is also lower because the optics of the attached lenses are of bad quality.
I do not recommend using them if you want your pictures to be better based on your front camera. However, there is another lens technology that can be attached to a phone which can make your photos better. Its price is steeper though. They are called
smart lenses and they connect to the phone via Wi-Fi.