the SB5101 is not a router. It's a modem.
Unfortunately, ISPs are limiting the use of docsis 2.0 for the health of the entire network.
Docsis 2.0 equipment is at their end-of-life. You will need to get a new 3.0 one. Upgrading to 3.0 will give you access to higher speeds tiers and a more resilient connection to congestion.
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SB5101N capped to 6mbps?