@Ihill 

Hi,

You can download the complete User Guide of the RE705X from the product webpage for the RE705X.

Anyway:

Thanks. I already checked the manual. It suggests that the signal is not good enough, but in fact I am getting a full wireless signal even with the red light on. And it is very fast. So I wondered why it might be on if I am getting a great signal!

  @Ihill 

Normally, that red LED should be linked with the signal indication for the wireless link between the router and the extender. It is enough if one of the two bands is "weak" for the LED to turn red.

Can you please double check that both are still showing "good" signal, despite the LED showing red color?

Yes there are three solid blue lights. Just the wifi light is red!

  @Ihill 

The lower two LEDs on the device itself only indicate whether there is a connection or no connection at all.

To see whether the signal of one or both bands is "weak" or "good" you have to look in the web interface of the RE705X.

  @Joseph-TP  I will try that out. But even with a red light I am getting a full wifi signal! my tether app says the extender has no internet, but I have no problems accessing the internet using the extender. Does it say that because it is not part of a mesh?

  @Ihill 

No, the message does not show because it is not part of a Mesh. More likely it's because of a weak signal.

Anyway, as per TP-Link's User Guide for the RE705X (screenshot posted previously) the red light indicates that the extender is receiving a weak signal from the router.

Since you are using the Tether app, could you please run the "Location Assistant" (accessible via the "More" menu) and check the result of this test? 

If the light on the RE705X is red, then I would expect the result to be like in the right picture.

  @woozle you are right. When I moved the extender the red light went blue. Tether confirms the strength of the signal. And when the light is red the lack of strength of signal.The intriguing thing is that even tho the light is red I am getting brilliant WiFi!! 

  @Ihill 

Well, if you are satisfied with the extender's performance as it is, then just ignore the red light.

In case the connection does become patchy at some point in the future, then you can still try to find a better location (i.e. another wall socket) for the RE705X.

Anyway, TP-Link will be glad to hear that you are happy with the performance of your RE705X.