Having same issue, can not access the checkout bag without getting
503 Service Temporarily Unavailable, I Do have Spectrum, I don't have a spectrum router or wifi, using the Netgear Orbi Router/Wifi.
Have talked to Spectrum, have talked to Netgear, and I've just finished talking to Apple.
The results were, Humm, must be Spectrum, must be Apple, and it must be the Router (netgear).
Bottom line, Six hours on the phone, and it just won't work unless I Tether my iPhone as an access point, then the computer will see that there is nothing wrong with the apple web site.
Apple wrote up a detailed notes, and will see if people up the chain can figure something out.
Have nowhere to turn.
AT&T is the iPhone network, for Tethering.