Most TP-Link models employ a security model that prevents the tplinkwifi.net page from being accessed if no clients log in to the management page for an extended period of time. Fortunately, restarting the modem or router restores access to the page. If you determined that this was the source of the problem but find this security feature annoying, you can most likely disable it by updating its firmware to the most recent version. TP-Link decided to remove this feature from most models, so you can remove it by downloading the most recent Firmware for your specific model.