I don't think we have enough information to answer this question:
It could be that the phone is not compatible with the 4G bands that your network uses.
It could be that there is just no 4G coverage where you are, or that there is a network problem.
It could be that your contract doesn't include 4G usage.
It could be that the phone settings are wrong (I don't really know 4G, but this sometimes happens with 2G/3G data).
Just knowing that it doesn't work means we can't know which of these lines of enquiry is the correct one. Hopefully listing these possibilities will be helpful to you in understanding the problem though.