AFAIK, A-Rival isn't a Chinese brand, but a German one (Baros GmbH ?). Maybe they let produce their devices in China though.
Your TEASI One is running Windows CE .Net 6.0 Core. Hence the "manufacturer" definitely told you nonsense: A device running Windows CE .Net 6.0 Core can't be hard-reset, as this was possible with former Windows CE 5.0 units.
FYI: The registry of Windows CE .Net 6.0 Core is file-based, it's not in ROM. So if something corrupted the registry, you probably have lost, unless you have a complete backup of Windows CE .Net 6.0 Core including it's registry.
IIRC, TEASI (A-Rival) provide a tool with which TEASI models can be updated/upgraded. This Windows PC program also allows you to play-back a backup you have made.
Note that your TEASI One's USB-mode always must be set to Mass-Storage mode, not ActiveSync mode.
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