First, is it a stick, a box, or a full-on TV? The model number should be on it somewhere. If you can use the Roku app on your phone as a remote you can find the model number under Settings->System->About. Finally, what are the first two characters of your serial number? We may be able to figure out what model it is from them.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.