4G licences were granted to three mobile network operators in Jersey in 2015, including targets for the roll out and performance of those networks. Links to the results of independent “drive tests" of the island road networks carried out by specialists Regulaid can be found below in the form of coverage maps for each of the networks by island.
JCRA's press release on 4G network performance can be found here.
The drive test maps for each island can be found below: