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When you reset the main router box, how long did you hold down the reset button?
And also, when your computers or other WiFi-using devices look for a network to connect to while within range of the main router, do they connect? If you haven't tried it, try turning Off the range exender, and just trying to connect to the main router. (The extender won't work if the main router doesn't.)
OK. So it isn't that the main router has had its setup wiped back to factory values, then.
Have you tried re-configuring the range extender from scratch? that is the thing to try at this point, since it may have either spontaneously reverted to factory settings, or simply had some of its settings corrupted.
(If you need basic instructions on doing so, let me know, and I'll provide them.)