FaceTime without WiFi

You do require a mobile data or WiFi connection to use Facetime. I presume you are trying on mobile cellular data and you are unable to use Facetime, correct?

Please firstly try checking Cellular data is turned on. You may also want to consider resetting your network settings, as this might also fix the problem. Otherwise, please take a look at the article below which will hopefully be of use to you.

If you can't make a FaceTime call on your iPhone, iPad or iPod touch – Apple Support

Not all carriers support using FaceTime on cellular. If you cannot make a call while on cellular, you need to contact your carrier to see if they support it or not. Only they can answer that question.