My data:

I am looking for a way to filter the dates using a custom date range. I\'ve s
You're trying to filter on date values, but you're ordering on type That's not how Firebase Database queries work: you always first order on a property/value, and then filter on that same property/value.
So to order/filter on regdate:
firebase.database().ref().child("Users").orderByChild('regdate')
.startAt("2019-01-05").endAt("2019-01-10");
Note that I also changed endat to endAt (with an uppercase A). The endat you had, will give a syntax error.
You're now ending at 2019-01-10. Since your actual values include a time, they will all fall right after this value. If you also want to include the users who registered on 2019-01-10 itself, use:
firebase.database().ref().child("Users").orderByChild('regdate')
.startAt("2019-01-05").endAt("2019-01-10 23:59:59");