cannot connect second extender ac750 to network

All I can tell you is authentification error shows on both extenders - it doesnt give a description of what it meanes. The router maybe doesnt like 2 extenders

Also problem seems to be linked with 2.4GHz band.

I can connect to the internet when 5GHz band is selected on ac750 - 0DF9A3_5GEXT

I used the same browser ms edge for the set up.

Is it possible by having to revisit the gene several times something could be corrupted?

Are you able to check from your end

Can everything be cleaned out and started again?

Looks like you're not the only one with issues adding extenders to that router. 

https://forum.vodafone.co.uk/t5/Other-broadband-queries/THG3000-Router-Setup-to-TP-Link-RE305-Extender/td-p/2647404

I see what you mean. Seems this router is the problem. It explains why I've been getting so many strange things occuring.

The only thing we now know is that only one extender will work on the THG3000

So now its a matter of returning things back to the way they were. N300 or AC750 on the 1st floor

Many thanks for your time & help. Pity we could'nt get a solution

Reconnected original extender N300 on 1st floor but could not connect to router. Is it possible I need to programme again after trying to set up ac750? 

I had not removed the N300 initialy when trying to set up ac750. Could this be the problem? Could one have effected the other.

Spent the last 2hrs on the phone with vodafone support. They advise router is ok following tests.

They are calling me back in 2hrs after I have contacted you for advice