I don't believe the Search option on the main Home Page menu is available globally, so depending on where the OP is located, he/she may not have this option. I don't know whether this also applies to the Search option from within the Streaming channels menu.
Oddly enough, the original Roku 2 (2720) is, as is the Roku Express 3700, but not the Express 3900.
Even stranger, the Roku 2 HD (3000), Roku 3 XD (3050), and Roku 2 XS (3100) are supported by Tubi, but Roku itself doesn't support those models.
I think Tubi has not updated their list of Roku devices. That matters because the app developer can pick and choose which devices to support. It looks like they aren't adding new devices to the list of supported ones. At least not in Canada.
If it was me, I'd contact Tubi. Or find an alternative to Tubi.
DBDukes http://www.dbdukes.com/Roku Ultra (4660) Roku Streambar (9102) Roku Streaming Stick+ (3810)