Default [Q] Galaxy Note II LTE locked to carrier

We were given a Samsung Galaxy Note 2 LTE by one of our customers. But we can't use it, as it is apparently locked to their mobile company, so it can't use a sim card for danish carriers.

I am told that the only way to fix this, is to root the phone. But what is needed and how do I do this? I don't need any extra features at all, just the lock removed.

I hope someone can help.