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Sometimes a simple power cycle of the router can correct the issue that you are experiencing. However, the issue could be a little more complex. Wi-Fi routers need adjustments to settings periodically for optimized performance with the constant progression of wireless device technology. Below, I have included two articles. The first article will cover basic Wi-Fi troubleshooting. The second article covers the recommended settings for current Wi-Fi access points.
Also, if you have a Mid 2013 MacBook Air, try the steps in this article.