Waist trainer showing indentation on buttock. What procedure would be best to improve this? (Photos)

It is not entirely clear as to why you are wearing a faja but I am presuming you are in the postop recovery period following Brazilian butt lift.? If this is the case, then the following information may apply to you:

You are in good company with many others who had high hopes for BBL but instead received disappointing results (lumpiness, unevenness, lack of projection in the central and/or lower buttock, post-deflation sagging, etc.) as most of the fat melts away. It is not quite as much the quality of the doctor nor liposuction technique but rather the quality of the procedure that is a large part of the problem. Thus, going back for round 2 BBL most often produces the same disappointing results but at another financial loss. Buttock implants indeed provide a more reliable, predictable, relatively safe, and permanent result, especially for adding projection to the buttock and often smoothing out lumpiness. Occasionally a lower buttock tuck may be advantageous for lower buttock fold asymmetries and/or sagging. Please keep in mind that an extremely few number of board-certified plastic surgeons specialize in buttock implants so be sure to consult with one that is reputable and experienced. Unfortunately you may have to travel long and far away from home to find this particular surgeon. Glad to help.

A lot of people have contour irregularities around their buttocks and often the back of the thigh.
This usually comes from connective tissue that anchors the skin to the underlying muscle fascia at a certain distance. If the thickness of subcutaneous fat is greater than the length of the connective tissue and indentation forms kind of like buttons on the tufted furniture.
There’s nothing people can do to treat these other than surgical intervention. Minimizing obesity is probably most helpful. Otherwise you can seek help from plastic surgeons though areas of indentation in this part of the body are often missed diagnosed and treated with fillers or fat transfer which often doesn’t work. excepting your body the way it is is probably your best bet as well as being cognizant of not becoming obese.
Best,
Mats Hagstrom MD