The operating system is in control of which SSID you connect to. It will, by default, select the SSID with the strongest signal.
SCENARIO: You have two SSIDs named "Network", one on the 2.4 GHz band and one on the 5 GHz band. You are connected to the 2.4 GHz "Network", but want to be on the 5 GHz "Network". When you click "Join" in WiFi Scanner on the 5 GHz "Network", you then select "Allow" then "OK", but nothing happens. You are still connected to the 2.4 GHz "Network".
REASON: The operating system is in control of which SSID you connect to. It will, by default, select the SSID with the strongest signal.
SOLUTION 1: The owner of the SSID can enable “band steering” on the router and it should redirect your device to the SSID on the 5 GHz band, using some 802.11 tricks to make operating system think only a 5 GHz network exists.
From wikipedia below:
“A technique called “band steering” is used by some enterprise-grade APs to send 802.11n clients to the 5 GHz band, leaving the 2.4 GHz band for legacy clients. Band steering works by responding only to 5 GHz association requests and not the 2.4 GHz requests from dual-band clients."
SOLUTION 2: If you cannot or do not have the "band steering" option, you can request that the owner changes the SSID name of the 5 GHz band to something different like "Network 5GHz". You're computer will treat the 2.4 GHz and 5 GHz SSIDs as two different networks and you will be able to join them independently.