I'm trying to extend the WiFi network of my home modem-router.

When I configure the RE705X via the web interface (tplinkrepeater.net) everything works as expected: I select the "extender" mode, detect the modem's WiFi network, select it, set the password (the same as the modem's, for convenience), and complete the procedure without getting any error or warning messages.

After that, the extender seems to work perfectly: from my tablet I can see the extended network, with a strong signal, and I connect to it without any problems, but then I can't access the Internet.

In other words: if I connect directly to the modem's WiFi from my tablet (as usual) I can access the Internet without any problems (although with a weak signal), but if I connect to the extended network (via the RE705X) I get no Internet connection.

I repeated the setup procedure several times, even resetting the extender, but the result remains the same.
So, at the moment, my RE705X is actually useless.
What could be the cause?
Am I missing something?