The T25 is an eMTA modem only - it is not a router. You cannot use both ports on the T25 simultaneously unless it's for Link Aggregation, or you've bought an additional IP from Comcast.
If you want to connect multiple devices to share your internet connection, you will need a router (and only ONE router) to connect to only one of the ports on the T25, and then connect all your other devices to the router.
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