They are the same
In this case, you can’t identify which device's Wi-Fi you are connected to. But only when you are connected to the extender's Wi-Fi, you can access the web page with http://tplinkrepeater.net. We have the following two suggestions:
1. Via Tether app
You can use Tether APP to manage the extender no matter which device you are connected to.
2. Via Extender's LAN IP
Log in to the web page of your main router and find the IP address of the extender. Then you can directly enter this IP address in your web browser to access the extender's web page.
For further help or technical support, please visit https://www.tp-link.com/support.