DIAGNOSIS: Your computer or phone is likely on a different network 'subnet' than the router. This often happens if you have another router (like a mesh system or an old router acting as an access point) plugged in incorrectly, or if the AX55's DHCP server got turned off.
FIX: First, ensure you're connected to the AX55's WiFi network (or directly via Ethernet). Then, find your device's actual IP address. On Windows, open Command Prompt and type 'ipconfig'. Look for 'Default Gateway'—that's your router's IP address now (it might be 192.168.1.1). On a phone, check your WiFi connection details. Type THAT IP address into your browser. If that fails, do a factory reset. With the router on, find the small reset hole on the back. Use a paperclip to press and hold the button inside for about 10 seconds until all the lights flash. Then try tplinkwifi.net again.
IF THAT DIDN'T WORK: Some security software or browser extensions (like a VPN or aggressive ad-blocker) can block access to local router pages. Try accessing the page from a different browser (like Edge if you use Chrome), and temporarily disable any VPN or 'security' plugins.
HEADS UP: After a factory reset, you'll have to set up your WiFi names and passwords from scratch. Also, if you had any port forwarding or special settings, write those down before you reset if you can get back in. The default login after a reset is 'admin' for both username and password.