First, download the latest firmware version for your router from the TP-link website. Make sure you get the right firmware for the version of your router mode. For example, if your router is v. 1 of...

How long ago did you purchase the router?

Have you tried their suggestion of updating the firmware?

NiGh7 wrote It wont go to RMA since it used to work properly. What? When stuff quits working under warranty you should be able to RMA it.

Play with channel width (20?, 40?, 80?) wifi generation protocol settings (n only?, g and n?, a, g, n?) in the router firmware. What version of the Chromecast dongle are you using?

You may need to create a network file share. Not sure how to do this in Apple OS but there are tutorials online. Since the USB external hard drive is behind the router I don't think the OS will see...