Optic Cable 5.1 Surround Sound

Folks, I need some help. I've got an audio receiver and a TV with optic cable ports. The audio receiver is DOLBY Digital compatible. My Onn Roku Smart TV doesn't seem to have the ability to play anything other than stereo sound through this output. It might be too old, I'm not sure. If I want to watch movies on this TV, on my speaker system in surround sound, what piece am I missing? Do I need to find a Blu Ray / DVD player with an optic output and connect to my receiver system that way? Is there a way to insure that it will actually utilize the 5 speaker system so I don't just have the same problem again? I apologize if I've left out any important information. Again, this is the first time I've set something like this up. The internet is full of AI articles and people debating HDMI vs optic and has been no help.