I have my own modem and it works well. If it is not working for you, you should try using tv.xfinity.com to watch live tv. If you have a problem watching "inhome" tv saying that you are not in home (I can't remember the exact message), you will need to call Comcast to get your modem properly registered. If tv.xfinity.com works, you should ask again here for more troubleshooting options regarding the network connection between the Roku and your home network.
That happened here last year, provisioning got dropped so connections were dropping at intervals until calling three times got it resolved.
Now no "in-home" streaming at all, wired or wireless, the My Account app shows no internet devices registered....
The xfinitywifi hotspots from other places also stopped working, telling me my service wasn't high enough. "Blast tier" apparently isn't enough anymore.
Their network administration is ridiculous.
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