I lost my 4g last week on holiday. Back home now and still no 4g when others have it around me.
Are the other people who can get 4G also BT customers?
You can try checking that 4G is still switched on:
You can check and make sure there are no issues with 4G in the area using EE/BT Network Checker
If all of the above do not help, you could request a replacement sim card via the sim section of your BT account at bt.com