Have you checked our Coverage Checker to see what the coverage is like in your area?
We have no reported known network issues at present.
Along with @DanielS 4G checker, you may want to check for any local outages or issues with mast in the area
BT/EE Network Checker you can check network issues in multiple areas
If you find that trying to iuse the handset in other locations also has the same issue you could try resetting the network settings, but this will also remove the wifi password, so make sure you have it to hand to rejoin your wifi network