@uberwaltz, Congratulations. You are making good progress. After AT&T installs the new modem, please ask them to run a speed test to confirm your speed is 75mbps. They should also confirm the cable signal strength is in the correct range. Please ask them to check your cable from from the wall to your modem and also your cable box outside. My Technician replaced the ends of my cable and rewired my inside wall box.
The speed and the cable signal test confirm the down stream components are working to specifications. My signal strength was out of the acceptable range and my neighborhood also had downstream components that were not up to spec. I suggest you ask the AT&T Technician to check everything else (this is what I did). The cable junction box near the street was corroded and was replaced. My cable work required an inside Technician and an outside Technician to complete this work.
I forgot something above. I suggest you re-boot your cable box and router around once per month. Unplug these devices for two minutes and re-start. It seems, that over time, these units need to be cleaned out.
And, after about one week, run the speed test again to confirm your service is 75mbps. Please keep us posted.