How do i truly stop this flea infestation?

DTE is a treatment and not a cure.
Do you know the difference?

You have to continue using it as a treatment until a cure is affected.
It will take time.
You'll have to use it until tge bugs die off and it has to go everywhere that fleas go in enough amounts that the bugs cannot avoid it.
If adults cannot feed enough to reproduce the cycle will break.
Take the dog swimming in cold water and while cold they can be brushed off better if done before they warm back up
Vacuum brush or combing device would help too if your dog can tolerate the vacuum, you can get a very long hose which would help.

This !!!!! I`ve been using just garlic powder for about a month and NO FN FLEAS!!!!! Just put it in their can food at least once a day. Use a lot ,don't be shy. I did a little research and fleas are vampiric by nature and HATE the garlic. After it(garlic) get into their bloodstream, they (fleas)will eiher starve and die or move on and find something else to eat (not you or your dog). Btw, mine are outside dogs and HATE to have a bath. Good Luck