After an extended comedy of errors (where do they GET those support people?) I now have a functioning Comcast gigabit cable modem. WiFi is giving me great speeds, especially compared to the 100Mb/s service I had until today.

For 5-6 years I have been using an ethernet cable, rated for direct burial, in conduit, to provide WiFi and ethernet connection in the shop. The run is about 250'
In the shop it plugs into a Tp-link gigabit switch and a cable from there goes to the shop computer.

Something doesn't make sense to me.

There are four ethernet ports on the the modem numbered 1,2,3,4 with a orange stripe painted beside #4.
Comcast tech support said it didn't matter which port I used.
I first plugged in the ethernet cable to the shop into port #1. The Ookla speed test gave me about 60Mb/s in the shop, even worse than with the old 100 Mb/s modem.
Comcast tech support said it was because my ethernet cable was too long. I didn't think so.

I tested the modem ethernet port #1 by connecting a laptop with a 4' cable. I got nearly 1000 Mb/s.

I then plugged the cable to the shop into modem port #2 instead of #1. Now I get nearly 1000 Mb/s in the shop!

For anyone still with me, is there any logical technical reason for the modem ethernet port #1 to be moving packets at only 6% of the rate on port #2?

The only thing I can imagine is a physical thing. Perhaps some wire in my ethernet cable plug is not making good connection when plugged into port #1, but twisting the connector 180 degrees to plug it into port #2 causes it to make a good connection. This doesn't give me a good feeling! Maybe I should cut off the plug and crimp on a new one. (I wired that cable six years ago and neither end has strain relief.)

BTW, I did connect a cheap Tp-link mesh node to the same gigabit switch in the shop to test the concept and it works well. I'm getting over 500Mb/s over the WiFi, more than I need for my iPad and phone!

JKJ