You can download the firmware from https://tvupdate.roku.com/ . You'll need the Roku TV's serial number (or really any Roku device serial number). I'm not sure of the Roku model number, but try 7206X. I downloaded a few Hisense 556+ firmware files with different Roku model numbers and they were all identical.
I'm not sure having the the firmware on a USB drive is enough since I think you need to be able to get to the menu to initiate the update.
You may have better luck with Hisense support.
Where to get support for your Roku TV™ | Official Roku Support
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