Strong signal but no service.

I have a very strong signal but absolutely no service on my phone  can't use it for anything. No phone calls, sms or Internet. Checked the coverage checker and it says strong 4G signal. I had a look on Three website and it says service will be slow due to work being done, it's said that for 6 weeks now. Its pointless using this service provider anymore as I can't do anything with my phone for huge chunks of my working week. Absolutely no way to find out how long the "works" will last. Can anyone advise me on how to find pit how long until its working again or if I can ask for refund due to no service?