DIAGNOSIS After a factory reset, the router broadcasts a default open network to facilitate initial setup. The TP-Link Tether app can sometimes fail to discover it if your phone is stuck on a different network or if there's a connection conflict.
FIX On your phone, go to Wi-Fi settings and manually connect to that open 'TP-Link_XXXX' network. Once connected, open a mobile browser (Safari, Chrome). It should automatically redirect you to the router's Quick Setup page. If it doesn't, manually go to 'tplinkwifi.net' in the browser. Follow the on-screen web setup wizard instead of relying on the app.
IF THAT DIDN'T WORK Your phone might be using mobile data, which blocks local network discovery. Turn off mobile data (or enable airplane mode, then re-enable Wi-Fi). Also, ensure your phone isn't using a VPN or ad-blocker that filters local traffic. As a last resort, use a computer with an Ethernet cable plugged into one of the router's LAN ports and access 192.168.1.1.
HEADS UP The factory default network is unsecured. Complete the setup process quickly to establish a password-protected Wi-Fi network. Leaving it on the open default network is a security risk and can also allow neighbors to accidentally connect and interfere with your setup.