And just like that, seemingly magically, with the router running the exact same firmware, Samsung services are now available. Bizarre. Maybe it was Samsung???

  @Riley_S Thanks for you reply. Yes, I did look at down detector and I would say what I saw there was inconclusive. I just can't come up with any good reason why Samsung web services would be available over one network vs another. Everything seemed to point to an issue specific to my network. Baffling to me...

Riley_S wrote

Hi  @axman53,

This does sound like it was some sort of error on Samsungs Side. If experiencing issues with a particular service, I would always recommend looking at sites like down detector to see if there is a widespread issue or outage being reported. Weird problems such as that can occur when Samsung is trying to send higher-quality content because you are on a wifi connection, whereas the cellular data will most likely mean that either your phone or Samsung is trying to save data by providing lower-quality content and images.