Can you hang a stick of EMT in the area of the lights, and run the cable through that? Or maybe reroute the cable? Looks like your original hunch was right, it was just the wrong suspect causing the interference. If the lights are on the same mains power transformer can as the modems, the interference could be coming through on the power side, as well. I get static on all my am radios that use wall power when neighbor turns on his damn security light.
aem sends....