The Pilgrims navigated a tricky tie at in-form Isthmian League outfit Carshalton Athletic and led 2-0 at half-time courtesy of Thanoj's 33rd minute free-kick and Luke Shiels' 37th minute header.
Christie Pattisson reduced the arrears during a sustained spell of Robins' pressure but Thanoj and substitute Jake Wright scored stoppage time clinchers to see the Pilgrims safely through.