There can be two reasons of this issue: cabling problem or faulty SD20.

 follow these troubleshooting steps:

In case of multiple OE01 distributor boxes, disconnect them from the line, leave only one connected. Make sure that the DIP switches are in ON position and there are no conflicts in display addresses.
Disconnect everything from the distributor, leave connected only the cable from SD20 to distributor and to one display.
If possible, pls check these cables with cable tester.
Stranded Cat5 UTP cable should be used, solid version is not suitable for RJ12 plugs.
Try to detect this single display, if detected, connect the next one, and so on.
If even one display cannot be detected, with a known good cable, try to use the other serial port of SD20.