EXT 6120 dual band authentication problem on smart tv

Hi. Ive just set up my wifi ext 6120 dual band as a range extender. It is working on all my devices beautifully, but my smart tv keeps saying authentication problem evertime I connect. I have rebooted the router, rebooted the extender and I have forgotten the network and tried to reconnect. The tv only runs on 2.4 but the 2.4 on the extender is on full range, so no problem there. As I said all other devices are running beautifully on the extender. I am not tech savvy and have tried as much as I can google. If you have any ideas it would be greatly appreciated, but please talk to me like im the learning three year old, as I am not great with tech.
Thanks :)