I would take a look at the spectrum and see what is there at that freq. When you unhook the sat. line does the modem link up?
If nothing else get some traps and on the line going to the modem filter out the sat feed. (expensive fix) Or make the customer pay for a new outlet installed. Or disconnect the sat completely and tell the customer to call for a service call and let the sat guys come do what they should have done the first time. Hooking up a spectrum will tell you real quick if there is something interfering with your downstream. It is up to the customer, Do they want to continue using your modems? If so they can pay you or the sat company to install a new homerun feed. Just my thoughts for what its worth bro.
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