Hi
As the Cellular Data options are greyed out when your main Cellular Data setting is turned on, Restrictions are likely enabled on your iPhone.
To turn them off (your Restrictions passcode will be required):
Go to: Settings > General > Restrictions > Cellular Data Use.
Note that it may not be possible to change the setting if restrictions are enforced on your iPhone (via a configuration profile, mobile device management or Exchange policy). If so, contact your device administrator for help.