macOS WiFi 2.4/5GHz

In case you have the same (B)SSID for 2.4 and 5GHz but you want to force your Mac to one of them it can be easily done with Terminal. First scan the network then using sudo join the selected BSSID followed by the channel. You must have joined the network and saved the secret to your keychain previously.

/System/Library/PrivateFrameworks/Apple80211.framework/Resources/airport -s
                            SSID BSSID             RSSI CHANNEL HT CC SECURITY (auth/unicast/group)
                      Fios-B5YRw b8:f8:53:2d:c9:63 -66  132     Y  US WPA2(PSK/AES/AES)
                           SINGH c8:a7:0a:9f:33:fa -88  132     Y  US WPA2(PSK/AES/AES)
                  BandidoBoricua 24:f5:a2:f0:3a:03 -87  100     Y  US WPA(PSK/AES,TKIP/TKIP) WPA2(PSK/AES,TKIP/TKIP)
                           3WV73 f8:e4:fb:0c:f8:db -72  11      Y  US WPA2(PSK/AES/AES)
                      FiOS-QT6C6 c8:a7:0a:9f:33:fc -70  8       Y  US WPA2(PSK/AES/AES)
                     xfinitywifi 16:4e:5a:1f:be:b8 -73  6       Y  US NONE
                           JWASH 04:4e:5a:1f:be:b8 -74  6       Y  US WPA(PSK/AES,TKIP/TKIP) WPA2(PSK/AES,TKIP/TKIP)
                      Fios-B5YRw b8:f8:53:2d:c9:62 -49  6       Y  US WPA2(PSK/AES/AES)
                  BandidoBoricua 24:f5:a2:f0:3a:01 -67  5       Y  US WPA(PSK/AES,TKIP/TKIP) WPA2(PSK/AES,TKIP/TKIP)

sudo /System/Library/PrivateFrameworks/Apple80211.framework/Resources/airport --channel=Fios-B5YRw132

Update: 2020-10-24

Unfortunately this change is not persistent