You need your router’s default private IP address to open its admin page and change Wi-Fi name, password, security, guest access, and network settings.
Various manufacturers and internet service providers give IP addresses to their devices. The more popular ones are 192.168.1.1 and 192.168.0.1, but there are a lot of other addresses also with regard to brand and model of the router.
Here are common default router IP addresses. If you know your router brand, use its default IP. If not, try the most common IPs one by one until the login page opens.