Do I need to use the tether strap when installing forward-facing?
Yes, the tether strap must always be used when installing the child restraint in the forward-facing position, whether using Lower Anchor Connectors or the vehicle seat belt. If the tether strap is too short to reach the vehicle tether anchor, you will need a tether extender, which can be obtained by calling ParentLink at 1-800-233-5921 (U.S.). (Pages 44, 49)
From the official Evenflo Symphony All In One Car Seat manual, page 44
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