Android
Reset Access Point Names (APNs) to Default auf Android
An Access Point Name tells your phone how to connect to your carrier network for mobile data, MMS, and sometimes hotspot sharing. If someone manually added a wrong APN, or a carrier update left a duplicate or outdated entry, Android can end up trying to connect through a path that no longer works.
Resetting the APN list removes every custom entry and restores the default profile that came with your SIM. This fixes most cases of mobile data dropping out, MMS failing to send, or hotspot refusing to start after a SIM swap or carrier change.
On a dual SIM device, make sure you pick the correct SIM slot before resetting, otherwise you reset the wrong profile.
Menu path
Tap on Settings ➔ Tap on Network & internet ➔ Tap on SIMs ➔ Choose a SIM card (Dual SIM) ➔ Tap on Access Point Names ➔ Open the Menu ➔ Tap on Reset to default
If data still fails after the reset, restart the phone once to force it to re-register with the network. If the issue only affects one specific app, check that app's own network permissions instead of touching the APN list again.