There is a host of possible reasons for why you are getting a “Server not responding” error message. To narrow down the culprit, try these steps:
First, unplug both your modem and Wi-Fi extender for 5 minutes. Then plug them back in and try to connect to the Internet.
If the connection gets lost and you see that error message, check for what stopped working - did the power lights on your modem shut off or start blinking? Or are the power lights on the Wi-Fi extender acting unusual? Is it your computer that stopped showing a Wi-Fi connection icon in the lower right-hand corner? Does the same error message appear in all the browsers you use (Firefox, Chrome, Edge, etc.), or just one? Does the same error message appear no matter what website you try to visit, or just one?
I know it seems like a lot, but once you know exactly what stopped working, you will know what to fix/replace.