We hope the following steps will fix your concern but if not, kindly contact ZTE Support Team
1. Disable other wireless connections such as mobile data, Wi-Fi, Bluetooth, GPS, and other connection when not in use.               
2. Dim the screen or turn off Auto-Brightness in [Settings] > [Display & Brightness].       
3. Place your device in a cool, moisture-free environment that's less than 32°C (90° F).

4. Clear all the background apps running that are not in use. Tap the multi-task or recent task key  located on the bottom left corner of your screen beside the home button and tap close.
5. Unplug the charger when the battery reaches 80% charge and avoid charging overnight.

6. Avoid using Bluetooth for extended periods of time and disconnect Bluetooth after using it.

7. Check the charging cable for any damage. Try swapping the cable to identify if the issue is with the cable. Always use the original charger that came along with the original box.