I’m so terribly sorry to have bothered everyone.
As is often the case, the problem here was the end-user / operator not keeping everything up-to-date.
The network-capable Arduino device being used here is the new UNO-R4-Wifi. An on-board ESP32 on that board handles the wifi / network communications, and like any microcontroller, it will have updates periodically.
Updating the ESP32 firmware to the latest version restored my SIDS upload functionality from this platform. For anyone who finds this thread and has the UNO-R4-Wifi, instructions for keeping that very important embedded ESP32 up-to-date are at:
… so now, a test payload transmitted over-the-air both decodes on my R4 -and- uploads to SatNogs!
… VERY sorry for the false alarm and I thank everyone for all the help past & present!
-Scott