Welcome to the Apple Community.
You can only locate your device when it is logged into iCloud and 'Find My Phone' is enabled, additionally the device will need to be switched on and connected to a wifi or cellular network.
Unfortunately, you cannot activate iCloud or 'Find My Phone' remotely.
If the device is wiped by you or another, you may not be able to locate the device (although some have reported they can)
If the device is unable to connect to a cellular or wifi network for whatever reason then all you can do is physically look for it.
You should also change the passwords on any Apple ID's you have in use on the device. If the phone is suspected to have been stolen, report it to the Police (if you have insurance ask for a crime reference number).