i don't think you want or need to "disable" the 2.4 band.
first, the 2.4 and 5.0 are not "combined". they are totally separate.
if all you want to do is to have yr iPhone always default to the 5.0 band you can do this well enough by:
1 naming the 2.4 and 5.0 meter bands two different names. such as yourname24 and yourname50.
you can do this renaming in iOS using the Airport Utility app available apple,or, using the macOS Airport Utility.
2 allowing yr iPhone to connect to the 2.4 network (which it might even not do now that you have renamed it)
but if it persists and defaults to the 2.4 then in Settings/WiFi/(network name) Info then do 2 things: 1 forget this network, and 2 unclick Autojoin.
3 then join the 5.0 band and click auto join.
no need to disable either band for those devices that dont have this problem.
even the iPhone Xs with this problem it seems to have can not join a network that it doesn't have the password for.