On a device that would require transcoding (old) or possibly a phone.This DiskStation?
https://www.synology.com/en-us/products/DS923+#specs
What specific "single" device?
Or just one device of any sort?
I am not sure about where transcoding fits in here - what are you trying to do?
What are the requirements: speed, resolution, etc...
Look at this review -- https://www.androidpolice.com/synology-diskstation-ds923-review/Can a synology ds923+ handle transcoding 4k content to a single device?