Return this one as well.
It should also be covered by the manufacturer's warranty. Even the remainder of the warranty on the original phone purchase should still be valid, given that you have had both phones no longer than 4 months. Verify the purchase date on the sales receipt or proof of purchase document for the phone to be sure.
If the seller won't help, consult the warranty statement contained in the documentation that came with the phone as to what to do to make a claim for a warranty repair or replacement.
If you can't find it then search online for ZTE ZMax Pro warranty for your country.