If you are in the UK.... The simplest answer is... ITS A NIGHTMARE
There are multiple posts regarding the topic
After many... one guy almost hit the winning post regarding Lte after many attempts.. The post is somewhere here
The starting point is, its not possible over the phone.. You have to walk into O2 shop in the UK (if you are with O2) and pray to god you are going to find someone with knowledge and experience
With Samsung... Life is not easy as Apple 🍏