Fixed! Go to the fix.
Yes, log on to the router, go to the Advanced section, Wireless and add 5 to the 5Ghz SSID, and untick the "synch with 5Ghz on the 2.4 section.
Then on the device in question get it to "forget" the original SSID, and connect it to the SSID you want.
Re: Dual band WIFI router
Not sure about 'force it', but I have a dual band Netgear, by my Firestick and providing access to the stick on 5Ghz and other access to rest of the devices on on the ground floor on 2.4Ghz. The Firestick is set to log in to the 5Ghz and always does anyway. None of the other devices have 5Ghz anyway and always log into the 2.4 access. I could also set the two passphrases differently anyway, which would force the matter.