Warranty claim instructions: https://support.roku.com/article/208757058
For a remote, if it asks for the serial number, they want the serial from the Roku you are using the remote with, not the serial number of the remote.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X