If you're connecting to the Router directly, you will be unable to access the Access Point's management page by using http://tplinkap.net.
If your extended Wi-Fi network name is different from the main router (e.g. MyHome on the router, TP-LINK_AP_XXXX on the access point), please connect to the extended Wi-Fi (e.g. TP-LINK_AP_XXXX) and try http://tplinkap.net again.
In case both Wi-Fi network names are the same (e.g. MyHome on the router and MyHome on the access point too), you may access the management page by using the Ethernet connection or access point’s LAN IP.