@martinz 

Hello，

Please put the RE650 and your main router in the same room, then refer to this FAQ to set up the RE650 on Tether app:

https://www.tp-link.com/en/support/faq/1373/

Note:
Please use your home wifi password to configure the RE650.
We suggest you customize the network name for the extender. You can see it in step 5.

If it still has this issue, please send me a screenshot of the error information, as well as the Model number of your router.

May it help and have a nice day!