Try charging from recovery mode...
With the device turned off and plugged in, press and hold the Volume Up and Power buttons until the ZTE logo screen appears, then release.
If it starts to charge:
Press the Volume down button to highlight reboot.
Press the Power button to select
If the battery is absolutely, completely dead, sometimes it won't even have enough power to instigate boot or a charge cycle. Recovery will *usually* boot as it requires less power consumption, and from there it may start to charge.
If this isn't the solution, it could be that the battery has reached its end of life, which would mean you would need to replace it, and from what I've read may now not be possible at all due to the manufacturer sealing the device after people had attempted to remove the battery having not realised it wasn't designed to be removed.
Personally though, I've never seen a device that the battery cannot be removed from with some tinkering. Unless they did something different with this one!