It appears to be device dependent, some have the initial online setup requirements, even the Community Forum on Roku’s site tells a poster they will need to setup with the internet connection and then can disconnect after setting it up.

Last post in this thread with a response by a former moderator.

You will need an internet connection and a Roku account to activate your Roku TV.

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Once the activation is complete, you can disconnect from the internet and continue to use your TV