Android gives users a lot of customization options, some of which are useful to everyone, while others are just for power users. One of these lower level settings is the Access Point Name-better known as the APN.
APN is something that most users do not have to worry about. You pop your SIM card in the phone, maybe restart it and connect it to your operator's network. You will be able to call, send messages, browse thank you memes and other important things. But if you're still moving around with custom ROMs, or if you're using an MVNO, you might find that things don't work automatically.
What is an access point name (APN)?
APN is All The information your phone uses to connect to your carrier's network. It lists the address the phone uses to connect to the network, the ports used to manage MMS messages, the type of data the APN uses, and other data to ensure the phone is functioning properly.  Some of the settings – such as "APN Type" – may be optional, and the phone still works without being 100% accurate. Others such as "MMSC" and "APN" are much more important, and your phone does not work if they are not entered correctly.
Fortunately, most phones have APNs for common carriers built in, so all you have to do is insert your SIM card and let your phone work its magic. This also applies to some prepaid carriers: I use Mint Mobile, which runs on T-Mobile's network. When I pop my SIM card in my Google Pixel 2 XL or my Samsung Galaxy S8, it only works. The phones already have APN, and it knows which one to load to connect to a particular network.
How to add or change your APN
If you do not need to change your APN settings, it is fairly easy to do. Start by opening the Settings app on your phone. The exact label of the menu varies, but tap which menu to handle your network settings. At Pixel 2 XL running Android 9.0, this is marked as "Network and Internet". Then press "Mobile Network."
Tap "Advanced." Scroll to the bottom of the menu, then select "Access Point Names."
Tap the menu with three points in the upper right corner, then select "New APN."
Your APN setting can be included in the package of your prepaid SIM card, or you may need to search for it. Enter the details just as they are listed, then press the three-point menu in the upper right corner and select "Save."
You may also need to make changes to an existing APN. To make these changes, press the APN you want to change. Tap each category and then update the information as needed. Tap the menu with three points and select "Save" when done.
The APNs that are pre-installed probably won't change, and if you open them to see the details, you'll see each object grayed out. Tapping them to change them will not do anything, so you need to create a brand new APN to change something.
When the APN is set, you do not need to replace it if you do not change the carrier or factory reset phone. Feel free to call, surf the net and much more!