I used to tell people a factory reset was enough. Handing your old phone to your kid or passing a router to your sister? A reset is fine. You trust those people, and the risk is basically zero. But ...
Go to Settings > Network & Internet > Internet (or Wi-Fi). Tap the network name and choose Forget, then reconnect with the password. Enable Airplane mode for 10 seconds, then turn it off. This resets ...