It's really difficult to be a true "full stack" engineer these days. I cringe whenever I hear it. You might be able to claim the title if you have 20+ years of experience and have grown up with some of the technologies in use today. But otherwise, I don't think you can find too many modern engineers that are really good at both front and back end. How many engineers are really good at both React and setting up EKS clusters on AWS? Not many. In olden times, you could be good at SQL Server and jQuery or Angular and call yourself full stack. Things are a lot more complex now. You can get really good at something, but it will always be at the expense of something else.
Anyway, if you really do want to call yourself "full stack," then you need to work on the frontend when they need you on the frontend, and the backend when they need you on the backend. Right now they need you on the frontend, so you work on the frontend. If that's not what you want to do, then first, stop pretending you're a full-stack engineer, and second, tell your manager that you would rather focus on backend.
Depending on the size of the company you work for, this may not go over well. If I were your manager, I would not be happy to discover that the full stack engineer I hired only wants to focus on the backend now. You might be the only frontend engineer that I have, and I might not have the ability to hire someone else at the moment. So it's you on the frontend, or you find another job and I hire someone else. Even if I were able to accommodate your request to concentrate on the backend, I would still very likely need you to do the frontend work until I find that someone else. In short, it's fine that you want to concentrate on backend, but that's not going to happen overnight. It's going to be transition. In the meantime, you'll work on the frontend.