Why do you want to unlock it? All of ours are unlocked but I’d hazard a guess that the operator has had a few spoiled loads due to drivers dicking around with temperatures so has decided to lock them out. If that’s the case then my advice would be to leave well alone.
If you miss the excitement of cycle sentry you could always pull into alternate lay-bys along your journey switching it off in one and back on in the next. 