First of all, you can read and follow the article below to understand the limitation of RE650 and to start the set-up
Why my Range Extender is unable to detect 5GHz wireless network of my root router?
However, your 5GHz Band won’t work if the bandwidth of 5GHz is not set to 40MHz. The image below shows Channel value of 36 and Bandwidth value of 40MHz.
The channel of your 2.4 GHZ Band can be “Auto”, but the band width should be 20MHz. If some of old devices can not connect, the channel can be changed to 1 instead of “Auto”.
Then, your 5GHz Band and 2.4GHz Band can be detected by RE650 with no problem.
You can read more about WiFi Channels and Bandwidth for better and deeper understanding.