RT-AC680 Extender mode - only 5Ghz band working

Hi I have a RT-AC680, I had it but never managed to get it to work as extender as I wanted. Yesterday I have given it an other go and discovered the 2.5Ghz band was the one not working, the 5Ghz works fine. Problem is I have a lot of devices connecting only to the 2.5Ghz band, so I am stuck and was buying something else. Thought posting something maybe as I have not found anything solving the issue. Every device I am connecting to the 2.5Mhz band claims there is no internet. I can not really do a lot with the troubleshooting of the bands once in extender mode. Maybe the router is faulty, just wondered if someone had a clue. Thanks