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Remember that, in configuring a range extender, one has to set it to the same SSID as the WiFi router one is trying to extend.
And also, the encryption code must be the exact same type (not just WPA, for instance, but if the main router is WPA-PSK, then the extender must be WPA-PSK too. This requirement may even extend to revision level, so if a router is very new and an extender is much older, it may not work.
Another point: range extenders must actually be within range of the main router. So check on that also, by trying to access the main router (after turning Off the range extender altogether, and unplugging it) from that same position with a wireless-equipped laptop, and see if it has a reasonably strong signal, of at least two bars I would recommend (though only one green bar *might* work reliably.)
Let me know how it goes, and what you find out, please, and how old or new the range extender is. Thanks.