Twenty minutes, in order
Your router is the one device every other device depends on, and it is usually configured once and forgotten. These steps apply to equipment you own or control.
- Update the firmware, and enable automatic updates if offered
- Change the admin password from the default, and store it in a password manager
- Disable remote administration from the internet unless you genuinely need it
- Use WPA3, or WPA2-AES where WPA3 is unavailable, with a long passphrase
- Turn off WPS — convenient, and repeatedly broken
- Put visitors and smart devices on the guest network, isolated from your main one
- Set a reputable DNS resolver and enable encrypted DNS if the router supports it
- Rename the network so it does not reveal your name, flat number or router model
Afterwards
Reboot, reconnect your devices, and then review the connected-device list. Anything you cannot identify is worth chasing down. Repeat the firmware check quarterly; it is the step that matters most and the one most often skipped.