If you can connect to your phone's hotspot just to get through the initial update, try that. You may have to factory reset to get back to the beginning (press and hold the reset button for no less than 30 seconds with the power connected). You can also try updating from a USB drive. You need to know the Roku model number (not the TV's model number). Hopefully you can find it under Settings->System->About if you can get there.
How to use USB to update your unconnected Roku TV™ | Official Roku Support
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