the beginning of the story
It was my fault! Monday's 28Mbps was not an anomaly. At one point in the installation, the WiFi hot spot crashed and I had to do a factory reset. Little did I know that factory reset disables the 4G radio: our throughput dropped to an abysmal 200Kbps. Once re-activated, 4G speeds came back and remain consistent.
After crowing success, then lamenting failure, I'm back to shouting "success!"