One good news is that you have a USB-TTL converter that works and that it talks to the Arduino.
Based on the above my first thoughts about the issue are either the option selection in the loop controller (compile time or run-time) or the serial communication parameters and maybe (but I don't think so) voltage level incompatibilities between the two micro-controllers (the Raduino runs at 5V, not sure about the loop controller).
I would do the following tests:
1. Connect the RX and Ground of the TTL converter to the TX and ground of the loop controller and use a terminal emulator which can display hex values. E.g. Realterm (see https://learn.sparkfun.com/tutorials/terminal-basics/real-term-windows).
Check that you see characters sent by the loop controller. That will tell you if the controller is sending the proper sequences, at the proper speed, parity, stop bits and so on.
2. Connect the loop controller to the Raduino and keep the USB/TTL Rx/Ground connections as above. That way you can "spy" on the Loop-to-Radio data transmission. Check that you can still see expected characters sent from the loop controller (eliminates issues of overload).
3. Move the Rx pin of the USB/TTL converter to the Rx pin of the loop controller and check that the Raduino responds to the frames sent by the loop controller. This would tell you if the first part is working, but not the processing of the responses.
That would give you a first pass at pinpointing the issue.