How do I enable the 2.5 Gb port on my Arris S33 modem?

What happens when you try to use the 2.5Gb port? Did you reboot the modem after changing which port is in use?

Forgot to add: What router are you using? And what about the Ethernet cable? If the router and modem cannot properly communicate a 2.5Gb handshake due to a bad cable or other incompatibility, that may prevent the port from seeming to work properly.

The port light flashes yellow when cable is plugged in.  I did not reboot modem after changing port.  The router is an ASUS AX3000.  The ethernet cable is good when plugged into the 1 Gb port.

So, try changing to the 2.5G port and reboot the modem to see if the router and modem make the proper handshake. Many people use the S33 with Comcast, so it should work. If you have more technical questions, ask here where there are alot of Comcast users with technical knowledge:

https://www.dslreports.com/forum/comcast

If your using a Asus AX3000 (AX58U) it only has 1G ports. It will do about 940/940. Over wireless about 500Mbps. So it don't matter what speed your paying for the AX3000 is limited to 1G. 

... Why would my laptop not connect to internet if I connect it directly into the modem? ...

Did you reboot the modem? As suggested above, you must always reboot a cable modem (reset button, remote reset, or power cycle) whenever you change the device connected directly to its Ethernet port before it will recognize the new device.

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