Tubi has its own channel/app:
https://channelstore.roku.com/details/7c34d32446def64db7eb2cc359749fe5/tubi-free-movies-and-tv
However, if you had it on your old TV and you linked your new TV to the same Roku account it should have been automatically added to your Home Screen. Is the email address that's displayed under Settings->System->About the same one you used for your old TV?
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