@Solla-topee 

Thank you for your response, much appreciated.

When I add "EXT" to the range extender's 5GHz network, I can connect easily, no problem.

But this is not what I want - I need the RE505X to have the same SSID as the router. And, according to this article (https://community.tp-link.com/us/home/forum/topic/160871), this should be possible to achieve.

Please accept my apologies for not making this clear in my original post.

Also, I would like to add that I have never used SmartConnect, as I prefer to set everything up manually.

And my topology is as simple as it gets - one AX11000 router and one RE505X range extender.

@Solla-topee 

Thank you for your response.

I'll follow your suggestion, i.e. set up a different network on the RE505X and change it to the router's SSID in a few days to see how things go. I'll report back in a week or so.

I was also wondering whether the new firmware planned for the AX11000 (with OneMesh) would not be a better solution to my problem. Here's hoping that it really does come out soon, I'd love to test it.

As regards the second RE505X, I plan to add it once I get to grips with the first one. Thank you for reminding me about the possible loop, however this should not be a problem, as the two will be installed on opposite ends of my apartment, well out of reach of each other.

@Solla-topee 

Thought I would write earlier, as I think I've stumbled upon a solution to the problem.

In September last year, when a new firmware was released, I set all of the bands on the AX11000 to WPA2/WPA3-Personal, and this indeed worked with all of my devices, including those that do not support WPA3.

However, it would now appear that the RE505X - which presently does not support WPA3 - had a fundamental difficulty with this arrangement, going crazy and not working properly when cloning the router's SSID. As an experiment (and out of desperation), yesterday I switched the router's security settings for 2.4 GHz and 5 GHz (1) to WPA/WPA2-Personal, version WPA-PSK/WPA2-PSK, and encryption AES, while leaving a fixed channel width of 40 MHz and channel number 11 for 2.4 GHz and a fixed channel width of 160 MHz and channel number 64 for 5 GHz (1).

I then connected the RE505X in range extender mode, cloning the router's SSID, and, lo and behold, everything connected beautifully and is working flawlessly right until now.

Obviously, this solution is not ideal, as you have to compromise your security, but I see no other option until the RE505X receives WPA3 support, hopefully in the not too distant future.