If you want to add or the APN Settings on your iPhone, but the menu option has been disabled by your carrier, we have a workaround for you. You can check how to manually change APN on iPhone
Normally the network or data settings (both apn and mms) are set automatically by the carrier in your iPhone, but sometimes they don’t work properly or there is a better and faster apn that you’d like to try. If you cannot access the Cellular (network) data option in your iPhone, you can try to Reset the Settings or Load the APN automatically via Safari.
How to Reset Network Settings on iPhone
To Reset the Network Settings on your iPhone, please go to Settings > General > Reset > Reset Network Settings – you will be asked for the passcode, enter it and confirm. The phone will reboot and will load the default network settings. Please note that this will also resets Wi-Fi network and passwords store on the iPhone
Load APN Settings automatically via Safari
The second option is to load the APN Settings automatically using an tool called APN Loader. For this, you’ll need to go to the tool (website) and select your country & carrier and it will install the network profiles for you. The online thing you need that the phone is connected to a Wifi Network. It works on any version of iOS.
Please check the below 2-minutes guide video for details:
After loading the APN Carrier Settings, please reboot the iPhone. You should then be able to browse the internet. If your carrier is not listed, please take a look at this post with an alternative option to update the APN on your iPhone