Android
Set Up an Access Point Name (APN) Manually auf Android
An Access Point Name tells the phone how to reach the carrier network for mobile data and for sending or receiving SMS/MMS. Android normally downloads this profile the first time a SIM registers on the network. If that automatic step fails, gets overwritten by a previous carrier profile, or the SIM was moved from another device, mobile data and text messaging can stop working even though signal bars look fine.
Adding the APN manually solves this by giving the radio stack the exact addresses the carrier expects. The setting lives under Tap on Settings ➔ Tap on Network & internet ➔ Tap on SIMs ➔ Choose a SIM card (Dual SIM) ➔ Tap on Access Point Names. From here, tap Tap on Add to create a new profile and enter the APN details supplied by your carrier, such as name, APN string, and authentication type.
After entering the values, open Open the Menu and choose Tap on Save. The new profile appears in the APN list but is not active until you tap it under Select the Added APN. Only one APN can be selected per SIM at a time, so switching back to an automatic profile later means repeating this last selection step.
If mobile data still fails after saving a correct APN, the issue is more likely a data limit, a disabled mobile data toggle, or a SIM registration problem rather than the APN itself.