Indeed, I have tried those things, as indicated on my post. Suspiciously, both my wife and I have this problem after upgrading to iOS 12.1. What I haven't been able to do is to try to send photos while attached to Wi-Fi which is definitely not Comcast. That is almost impossible in this town.
Could be my Comcast modem, but that was replaced about 20 months ago. Seems like those kinks should have been worked out long ago. Regardless, this is a pretty poor showing for both companies. Do they not learn? Why would this problem show up again after 6 years? Did it ever really go away? This is the sort of thing that would have gotten me or one of my staff fired back in the day when I worked in IT. Yeah, I only worked at smaller places where we had to face our clients directly on a daily basis. I'm guessing that is part of the problem here. The people who actually write the code at Apple and Comcast are insulated from angry customers, not to mention from problem reporting. As if this problem could be reported to either Apple or Comcast by an end user. (To be fair, it isn't just them, as Microsoft proved with their failed October update.)
Please recognize this for what it is - a rant as much as a cry for help. I'm graduating in January and rushing to complete my requirements. Unless I hear of an easy fix, I'm unlikely to do much about this before February, at which time I'll probably run the phones into the genius bar and let them figure it out. I don't have the time.