[Darryl Ramm]

Break the problem down into trivial pieces and troubleshoot.

Get a mono "piece of wire"/cable insert working first and confirm removing it breaks the signal flow in Pro Tools.

If needed, drive a signal generator from Pro Tools into the effect box with a powered monitor on the output of the box and confirms it works as expected.

Everything you should possibly need to debug this is right in front of you or worse case you may need a few more cables. There is not much more folks here can do, this is not in front of us. Solving these kind of things usually comes down to some experience but mostly the willingness to jump in and troubleshoot stuff, so I definitely want to encourage you to do that.