Oppo
Turn On Automatic Network Selection auf Oppo
Oppo phones running Coloros normally scan for the strongest available carrier on their own. That behavior sits behind a toggle called Auto select, which lives inside the SIM card menu rather than the general network settings.
Once someone switches to manual network selection - often to force a specific carrier abroad or to work around a weak signal - the phone stops re-scanning by itself. It stays locked to whatever network was picked manually, even if that network drops or a better one becomes available nearby.
To bring back automatic scanning, open Settings ➔ SIM card & mobile data. On dual-SIM devices, pick the SIM you want to fix under Choose a SIM card (Dual SIM), then tap Carrier and turn on Auto select.
Leave manual selection in place only if you deliberately need one specific carrier, for example while roaming on a partner network. In every other case automatic selection gives more reliable coverage and avoids "No service" issues after moving between cell towers or crossing borders.