You have to be sure you are installing and comparing the same type modems, the CM1000 is docsis 3.1 and you don't specify the models for the others. I had a similar problem with an MB8600 that supposedly failed and it took Comcast nearly 7 months to figure out what had failed in their local plant. I went through multiple MB8600's and two CM1000's and they were still sure all those were bad. My old docsis 3.0 modem still worked. Log into your modem and post the status and log information.
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