How did you test the speed?
Can you reset ER5120 to factory defaults and reconfigure it to work with your new modem?
Later, connect just one computer to ER5120 and test the speed again.

Tony Seaford wrote

How did you test the speed?
Can you reset ER5120 to factory defaults and reconfigure it to work with your new modem?
Later, connect just one computer to ER5120 and test the speed again.

I tested the speed with both TWC's speed test and with SpeedTest.Net. I confirmed the speeds by monitoring it through the ER5120's built-in monitor.

I have tried resetting once without any change, but I may try it again. I'm hesitant because I'll have recreate a bunch of rules.

Before I do that, I'm very curious to confirm whether the v1 hardware can really achieve the reported 345 mpbs throughput.

UPDATE: After some more fooling around, I'm getting 200+ mbps down. I had made some changes yesterday but was testing the speed at my main computer (which is connected to the router through several switches). When I checked again directly at the router, I was at full speed. I think the key was actually getting TWC to reset the modem and check the provisioning.

I'm so happy. I've always loved this router because it's so darn stable.