This solution solved the ZTE USB modem connectivity issue. Many people have said that this solution has solved their problem.Since it is a USB modem issue, download and install this HiLink.zip and check whether your problem is solved or not.The procedures to be done are given on the website page.Download and install this module HiLink.zip.Once you have done that, your Mac will be restarted.If you left any of the windows unclosed, a pop-up appears to close the opened window or apps.Click on the Apple Logo on the top left side of your mac.Since it is related to USB Modem connectivity issues, so you can try this method and solve your problem. Restarting method will solve most of the common problems that occur in Mac. The First and foremost solution is restarting your Mac.