New CM1200 modem download speed capped at 100Mpbs

[Grunt66]

Last Sunday (10/17/21) I replaced my Surfboard SB8200 modem with a Netgear CM1200. I had been using the SB8200 since early 2017 when I upgraded my Cox Internet service to 500Mpbs download. The reason for the modem change was my internet connection was having intermittent connection issues on Saturday and Sunday. I thought I trouble shoot with the modem first since I replaced my router earlier this year. The install went fine and everything was working great until Tuesday. It was then I noticed the LAN light on the front of Modem was amber instead of green. Which per the manual for the CM1200 indicates a device is connected at 10 or 100Mpbs. A quick speed test resulted a download speed of 98Mpbs. I used Ookla and Cox for the speed test and both tests comfirmed the speed was capped at just under 100Mpbs download speeds. I reset the CM1200 several times and the amber light remained lit and the download speed was still at just under 100Mpbs for download speeds. I called Cox support and they reset the modem on their end and it fixed the issue, the LAN light turned green  on the modem and my download speeds where once again at 570 - 580 Mpbs download speeds. All was good until yesterday evening when I came home from work and noticed the LAN light on the CM1200 was once again amber instead of green. The download speeds once again now capped at 100Mpbs and a speed test resulted in 98Mpbs download speeds. I first reset the modem several times and that did not work. I called Cox and a reset on their end did not work. I connected the SB8200 back into system and my speeds are now over 500Mpbs download. I am thinking about returning the CM1200 and buying another SB8200 at this point since I cannot keep the CM1200 with download speeds over 100Mpbs. I made sure LAG was disabled on the CM1200 and I am using CAT 6 ethernet cables and my PC is hard wired for connection.  Any thoughts on this issue would be greatly appreciated.  

[Grunt66 · Accepted solution]

The best option was to use the Arris SB8200 and it works flawlessly. The CM1200 was actually cheaper since I am retired from the military and use the military exchange (retail sales without sales tax) aboard the military base. So the CM1200 was returned and I am using a new SB8200 and download speeds are a consitent 570Mpbs and the light on the front of the SB8200 remains a constant blue color instead of green. I really wanted the CM1200 to work out. 

[plemans]

did you try replacing the cable? I've seen sketchy cables have issues and result in the 10/100mbps connection. 

[Grunt66]

I replaced the CAT 6 cable with a new one and even tried another new CAT 6 cable ( I have various diiferent new CAT 6 cables for other projects) and the issue was still there. I also removed my Router (Asus GT-AX1100) from the equation and connected the modem directly to my PC and the issue was still there. Intresting enough the SB8200 is displaying a steady blue on the front panel indicating it is in 3.1 mode and all tested download speeds are in excess of 500Mpbs. 

[plemans]

Do you have a screen snip of the cable connections page and the event logs? 

I've seen ISP's assign different frequencies at times which is why sometimes a reset/reboot/ISP reboot can fix issues for a bit. And then later it goes back to the frequency that is having issues. 

[Grunt66 · Accepted solution]

The best option was to use the Arris SB8200 and it works flawlessly. The CM1200 was actually cheaper since I am retired from the military and use the military exchange (retail sales without sales tax) aboard the military base. So the CM1200 was returned and I am using a new SB8200 and download speeds are a consitent 570Mpbs and the light on the front of the SB8200 remains a constant blue color instead of green. I really wanted the CM1200 to work out. 

[FURRYe38]

Next time, would have been good to see a screen capture of the Connections status page and Event logs to help better determine what may have been happening. Glad you found something else. 

Enjoy.