I got this to finally work. Not sure what the critical step was, but here are the steps I took and the intermediate results.
I started by reading this prior post: Error when applying radio firmware update on RV50 which, as it turns out, I had commented on at my prior job.
As I looked it over, I was reminded that a recover reboot also wiped all the other files outside of Aleos, which included the radio firmware. On the wild assumption that the firmware was being loaded from someplace wrong, I ended up attempting the recovery boot and installation of Aleos 4.16.0.021 from https://source.sierrawireless.com/resources/airlink/amm/gateway-software-packages/
At the end of recovery reboot, I had a Red/Yellow flashing Network LED and a Red Flashing Signal LED. According to the RV50 Hardware manual, this is due to Network Switching being enabled but the firmware is not found. That is what I would expect given the recovery should have wiped all the extraneous files.
I logged in, changed the password, then navigated to the Admin → Radio Module Firmware → Firmware. The list of firmware was complete empty. I uploaded all the radio module firmware for North America from the above 4.16.0.021 tarball of firmware. When I was complete, I could see firmware for ATT, GENNA-UMTS, SPRINT, and VZW which is what I would expect. The radio LEDS remained unchanged.
I power cycled the modem. At this point, I had both Red Flashing Network and Signal LED. This was confusing as it seemed to indicate it was not receiving a strong signal. To ensure there was a strong signal, I attached an direction external antenna pointed directly at my provider’s tower. No change.
I then attempted to force a network operator switch by loading the ATT firmware by using the ‘Software and Firmware → Radio Module Firmware’ upload. It warned me about network switching, but allowed the upload. Once it rebooted, the Network LEDs went through a series of changed flashes, but after about 2 minutes, settled on solid green for both Network and Signal. I was back in business.