My radios are working now, so thank you for your concern Igor and Egor, and I’m sorry I don’t know what the real solution was in the end, but here are some related (and unrelated) things I did or noticed while troubleshooting:
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upgrade to latest RFD900 firmware
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moved the serial cable to the wrong DFU connector for a time (oops, should be beside the usb connector, not on opposite side of board)
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on one computer, Reach’s USB to ethernet is now being recognized as enx220555fb0af7 instead of usb0
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one Reach was crashing server.py with solid red LED, it turned out to be an empty /lib/systemd/system/bluetooth.service file. Weird. So I copied the file contents from the other Reach. edit: I think it was a bug, fixed here.
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used the program screen in Reach to connect the console to the serial port like so: screen /dev/ttyMFD2 57600 and with that, I was able to:
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type characters on one Reach, and watch them appear on the other Reach, and
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send modem (AT) commands to the local and the remote RFD900 modem
Next, it would be nice to have an easy way of changing the transmit power level (dBm) and air data rate of the RFD900 modem while in the field.