Welcome to the community! Here are a few things to consider.
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With the Harmony Hub, 95% of the devices controlled are through IR. So, placement of your hub is critical to its success. Make sure that all the devices that are being controlled by it have a clear line of sight (even if the signals are bouncing back from a wall). For instance, I have my living room one setup under my TV and then I use the IR extension (one (or two? I don’t remember) should have come with your hub) to control devices in my media cabinets. The Harmony Hub has IR blasters at 6 points inside its case, so try to make sure that all your devices can be hit by at least one of the blasters.
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Personally, I’d absolutely get rid of the Spectrum router and go with your own. Not only will you save money (leasing versus owning), you also have complete control over your local network. The routers you get from ISPs aren’t exactly known for their features and/or reliability. Go with your own and, if possible, place it as far away from your other hubs as possible.
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As for the Digital Life components, I’d get different ones. There’s been some work on getting AT&T Digital Life components working natively with ST, but I don’t know what the outcome of that was. Personally, I’d go a different route. Let Digital Life be your security component and then for the smart home stuff, go with brands that work with ST natively. You can usually get the Iris smart plugs on sale (from Lowe’s) for ~$25 with a coupon code. Then, once you do that, you hook that up to your ST hub, name it, and have Alexa discover devices. This will allow you to say “Alexa, turn on the fan” without having to use a skill.
Lastly, I’d HIGHLY recommend reading both Bulbs, switches and sensors, oh my....what to buy (device class features) and FAQ: Networking and Reducing Channel interference between WiFi and Zigbee. They both go into much greater detail about networking and devices that work best.