I doubt that back panels are the cause, though I wouldn’t rule it out as a possible cause just yet. That would be more likely than the main board anyway.
The first check is to see that the COM of the tester matches the COM ID set on the 850-DS. Also make sure that the 850-DS is set to App 1/2/5/6 and not App 3/7.
If you can get to the 850-DS diagnostic screen where you can control the tester then you have communication. Otherwise you cannot even reach that screen.