What's the best treatment to help get rid of these deep depressions and cellulite on my buttocks? (Photos)

Thanks for the question. 
In this regard, I recommend you waist and back Liposculpture, getting better curves especially in the waist, improving your body contour.
Finally, I suggested take the fat we get and transfer to the buttocks and hips, to mold them and get better volume, contour and butt projection, is that we know as "Brazilian Butt Lift" (BBL).Kind regards

Unfortunately you have problem with cellulite which is not going to be significantly improved with fat transfer or liposuction. Postoperative swelling after both procedures may improve appearance for a short period but in a long term you will not see significant benefits of these procedures on cellulite. Good luck.

Superficial fat grafting to dents on the buttocks can be performed. In this process, fat is liposuctioned from other areas of lipodystrophy and transferred in very small aliquots to the dents. This is never a perfect solution to the dent problem, but it can improve the areas in most instances.

Dear insideout25,
You might consider Cellfina to reduce the cellulite. Patients are usually satisfied after the treatment (over 90 %). I would suggest you to consult with a board-certified plastic surgeon with experience with Cellfina.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

Hello dear!
Thanks for the question and provided information as well.
Based on your pictures, you are a good candidate for liposuction with fat transfer, to improve your buttocks and fill the dents, and I think you'll get very good results. 
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)

hello thank you for contact and fotos, the best way for you is perform a liposuction and filling dents with fat. That become your butttocks in the best part of you, rounded and shaped your body.

You would be best served with filling the dents in your buttocks with fat.  You may also require releasing of any bands holding the skin down at the same procedure (all performed through tiny incisions).The leg crease/dents would be better off treated with liposuction to smooth and thin the areas.Dr T

In order to completely eliminate your cellulite on your buttock, posterior thighs, lateral thighs, we will have to tuck your skin using a lateral thigh and buttock tuck. This is because cellulite is created by skin redundancy that needs to be eliminated. Please contact a high definition body contouring specialist to appreciate what is possible.

I am sorry to hear that you have had previous treatments for the depressions without improvement although I'm not surprised. It appears that the linear depressions are not sharply defined but rather shallow soft depressions. while it would be important to examine you I believe that you can benefit quite well from fat transfer which can be performed under local anesthesia. Good luck!