It's a general Lumia Hash Code. Some firmwares have it locked out.
The RM-994 should have the correct LTE Band on the radio for O2 or O2 airtime reseller in the UK. Getting into Field Test would let us know if the firmware disabled LTE Band 20 and allow it to be re-enabled.
If you dialed the whole code (including # symbols), ##3282#, and it did not open Field Test then you are locked out. Not every variant allows access to Field Test.
Contact Tesco and make sure your account is authorized for 4G and see what other troubleshooting they can help with.