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Please update your specs as the Arris 6190 does handle the gigabit service. Cox says it does not go above 300 that is indeed incorrect information. As Cox would be the issue not the Modem please update your customer service.
The ARRIS Surfboard SB6190 is a DOCSIS 3.0 so it wouldn't support 1 Gbps. It's only rated for 300 Mbps / 30 Mbps.