I will start with the router with WPA2 or WPA3 encryption. The older models uses WEP encryption. Check for any update of router firmware regularly to protect against security vulnerabilities. Do not use the factory default passwords for your smart devices, set a strong, complex and unique password and it is better to enable two-factor authentication (2FA) for extra layer of security, this will prompt for a code to access the device. There are many 2FA apps such as Google Authenticator or Authy etc. There are many VPN that offers secure, encrypted connection for privacy purpose.
Remember to reset it to factory settings to erase personal information before disposal