Every carrier uses its own APN Settings to get its 3G/4G LTE network working on an Android smartphone or tablet device. Now, these APN settings are usually "downloaded" automatically to your phone when you insert a SIM card. BUT, there are times where the APN are not selected automatically OR when the APN settings are not downloaded.
In those instances, your 3G/4G LTE data will not work but that can easily be fixed.
Step 1. First, browse your Settings menu and find the "Access Point Names" menu. This is usually under "Mobile Network Settings" or "More Mobile Networks" or "Mobile Networks" under Settings menu depending on the Android device used.
Step 2. Once you are in the APN Settings menu, first thing you should check it that the APN is on the correct one. If the correct APN is not checked (sometimes this happens), simply check ON the correct APN for your carrier. That will make your 3G/4G LTE data working right away.
Step 3. There are also times where the wrong APN is listed or there's simply no APNs listed. In these rare cases, you can simply add a new APN, hit the "+" button.
Step 4. For APN setting, the "Name" can be anything. "APN" setting is the most important, usually you only have to fill this out for your phone to work.
Step 6. Hit "Save" when done.
Step 7. Next, make sure to select the new APN Setting you have saved. Within 5-30 seconds, your phone should start picking up 3G or 4G LTE signals and your data should be working again on the phon