Hello   @cloggy 

Can you share a photo showing the error about cookies? May I know what web browser are you using right now. 

1. This error seems to relate to the web browser, you could try a different web browser like Chrome, Firefox, Edge etc. Or you could try to open web page on a different network device like phone or PC. 

2.  Try to turn off the Private/Incognito window or enable cookies of your browser.

Take Chrome as an example: 

In the address bar of Chrome, type in "chrome://settings/content/cookies";

Then you will be able to see a switch that could turn on/off the "cookies", please turn it on.

Hello   @cloggy 

Thanks a lot for more details.

May I know does the login page pops up as soon as you connected to extender's wifi SSID? If so you could close this page and try to set it up on Tether APP. (phone is connected to extender's wifi network so Tether app can find RE ) Note RE220 V2 and above version support Tether APP.

It looks like the RE web page is opened via browser on the phone. 

1. Kindly check if there is any incognito mode or privacy mode in the phone browser to turn off

2. On the right top, try to tap' 3 dot' and see if you could open the same page using a different browser, instead of the default browser

Or open a different brower and tapo ' tplinkrepeater.net' to access web page (phone connected to extender's wifi )

3. Try a different phone or laptop to setup the range extender 

Feel free to let us know how it goes. btw, what is the model and OS of your phone?