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registration of modem fails

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registration of modem fails

After 4 days of trying to talk to someone in tech support I've decided to give the forum a try, xfinity chat on website doesn't even work, so much for comcast trying ot rebrand itself with a better image. 

Anyways, venting aside.    I plugin an additional modem in coaxal outlet and am picked up on xfinity network and firewalled, when i go to register it message says something went wrong.  All I'm trying to do is add the modem to the LAN, my son's modem, another secondary modem was added by someone in tech support and it works fine as well as the primary modem on the account.  I'm not sure we have all this configured correctly, but what is the recommended configuration or procedure to use additional modems in the home and have it recognize the one default gateway, which is the account modem.  I'm just trying to get us hardwired within the Home LAN through the coaxal lines that split from attic?