Officially it seems you need a licence or feature key to obtain narrow band only mode. That is why it is grey non function.
Reference: Hytera DMR Radio Authorisation Operators Guide page 2 Rev 2.0 August 2017 : Added Only Narrow Band, Recorder and SFR features for the authorisation.
And below from the CPS help file on the Feature Control Tab.
Only Narrow Band
This option decides whether the band width of radios is only narrowband.
- Checked: The bandwidth of the radio is only narrowband.
- Unchecked: The bandwidth of the radio is either narrowband or broadband.
Unchecked
This feature is available only for some areas, for more details please contact your dealer.