My Echo will only connect to the 5G band too and only if the security is set to Open, with no password. I have received an email from Customer Services at Amazon with details of the adjustments needed to be made to the router to enable my Echo to work on the 2.4 band and with a secure network.
I can either phone the router tech services to see if these adjustments can be made or get a new router!
Thanks for replying folk!