Roku warranty claim instructions: https://support.roku.com/article/208757058
Note that remotes do not have serial numbers. When serial number is requested, Roku suggests you enter the serial number for the Roku unit you are using the remote with.
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