Have you made your app unique or you just reskinned one of the popular app sources, such as Pocket Reward App?
If it is a reskin, you will have difficulties, because Google suspends most of these type of apps.
If it is unique, you can try to promote ir using Adwords or contacting youtubers, who do such app reviews. You can get many free or cheap installs from those. I have done it with my reward apps in 2016-2018 and made good money.