Sometimes the carrier settings are outdated. Go to settings and check for a "System Update" or "Carrier Settings Update."
GSM is vulnerable to false base stations (IMSI catchers), A5/1 encryption cracks, and SS7 exploits. get gsm tips
Use the built-in battery optimization settings to turn off background data and reduce brightness. 4. Advanced GSM Tips: Firmware and Technical Maintenance Sometimes the carrier settings are outdated
or pry tools to open casings without causing cosmetic or structural damage. A5/1 encryption cracks
Android: Settings > System > Reset Options > Reset Wi-Fi, Mobile & Bluetooth. Physical SIM Card Maintenance