Samsung
Set Up Access Point Name (APN) Manually auf Samsung
The access point name, or APN, tells your Samsung phone which gateway to use when connecting to your carrier's mobile data network. Most devices receive this setting automatically the moment you insert a SIM card and connect for the first time. If that automatic setup fails, or gets deleted by a factory reset, a manual SIM swap, or a system update, mobile data stops working even though you still see signal bars and can make calls.
Where to add or fix the APN
Open Tap on Settings ➔ Tap on Connections ➔ Tap on Mobile networks ➔ Tap on Access Point Names. If the list is empty or shows the wrong entry, select Tap on Add and fill in the corresponding data your carrier provides, such as APN name, MMSC, and proxy values. Once every field is filled, use Open the Menu and confirm with Tap on Save.
When this fix is not enough
If mobile data still fails after saving a correct APN, the SIM itself may be damaged or not activated on the network side. In that case contact your carrier before changing settings again. Rebooting the phone after saving the APN clears cached network profiles and applies the new entry immediately.