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I would like you to check if the antenna connectors are screwed properly.
Depending on the environment and surrounding objects, try changing the placement of the router.
Check your configuration by following this video.
Also, login to the WebUI and navigate to Network -> Wireless. Edit the global wireless device settings and in a General setup tab try setting channel width to 40MHz. More details here. You can try different channels if you are in an area with more wireless devices around.
If the above does not help, I would like you to attach a troubleshoot file to your question. Please, replicate the issue, then access router's WebUI, go to System -> Administration > Troubleshoot section and download troubleshoot file from there.