I am now in two minds, should I take a step back and not do my job at the expense of keeping them happy or should I just do my job at the risk of job security?
Your boss expects from you that you do both, and that is also what I would expect from a scrum master.
Unfortunately you can't just come to a new team, tell everyone how they have to work and they will just do it.
As a scrum master you are not the boss of the team, you are the servant leader.
I have spoken to my manager and his attitude is that I should focus on being really liked by accommodating their needs which is a problem since by doing that will prevent the team from doing agile properly.
Have a conversation with the two team members and be open about what you want to achieve and that you need their help. Ask them what parts of the process they are objecting to and what they can agree to.
Explain why you want to change the process and what you are hoping to see from the change. As management brought you in they were clearly looking for something here, make sure that you know what this something is and your approach solves this.
This can go like this: "What I am trying to achieve here is to get more predictable results. Management thinks that this needs to be improved and I believe by breaking bigger tasks down and assign story points, can have more accurate estimates."
If the individuals don't agree that there is a problem, I would schedule a meeting with management to make sure everyone is on the same page here.
When you agree about what the problem is try to figure out why the team members are resisting your solution and what alternatives they propose. Could these alternatives work too? Why not give it a try, there isn't only one way to do agile properly, agile is doing what works.
If trying their way doesn't work, (make sure you have a clear success criteria) you can propose doing it your way.
Don't try to introduce all your changes at the same time, start with the most impactful one and after the team gets used to it and trusts the process more reflect back and suggest more changes.
If you encounter a lot of resistance, document what you have tried and why it didn't work. This is your last resort, as it might be that the team members in question are completely unreasonable. Then, and only then, you can take this evidence and bring it up with your common managers.