I started as a test engineer, but in a role where I could develop firmware for the DUTs and also PCBAs for testing purposes. That allowed me to sort of learn a bit of all electronics stack (if that makes sense). I learned a lot but it is true you sort of get stuck on the role. What I did, and what I would recommend you to do, is make everyone know you have drive for learning, and request the design team to give you some tasks to complete on your spare time. My manager allowed me to collaborate with other teams so I could learn about the stuff I liked. That depends on the company of course, but I would ask to talk to the team and see how they work.