Hi @Suzi Kolpin,
Since starting in safe mode didn’t change anything try a hard reset and check if that resolves the problem.
Be aware that a hard reset will erase all your data and downloaded apps. It will restore the phone to its’ factory default condition. If you wish to try this then backup the phone to the cloud or a PC before performing the hard reset.
Backup the phone using the phone’s backup feature.
When the backup has been completed, perform a hard reset (aka factory restore, master reset) using the phone’s factory restore feature or by using the hardware keys as shown in the link above.
Once the phone has been reset, check if it operates correctly.
If it doesn’t work correctly when it has been reset then you have a hardware problem.
Has the phone been dropped or gotten wet at all?
Here’s a link to the ifixit ZTE ZMax Pro Repair guides which will help you to open the phone and inspect it for any loose connections or obvious damage.
If it works correctly when it has been reset, using the backup that you made earlier, restore the phone using the phone’s restore feature.
Once the phone has been restored to what it was, check that it still works OK.
If it still works OK then the reset has resolved the problem.