I think to be able to go 4G+, the device must also support it. I remember the old Samsung s4 has 4G and 4G+ version. (May be same for Samsung Note 4, as rear cover of phone got print 4G+).
Ie it may be more then just the required LTE frequency band.
Yes the device must support CA which "combined" 2 or more LTE band to achieve. Maybe Xiaomi device the combination LTE band doesn't support our local Telco LTE bands.