Look at the cable and make sure there's no stress cracks in it and it seated tightly on both ends. Some just slide on so could be a poor connection between the lnb out and the module. If you had another short piece of cable could just swap out to see if you get a signal then.
If you want to see if the stb will work on 91 without the module as a test turn the stb off with the back switch. Unhook the loop out cable from the lnb out and the module. Also probably be best to open the case and inspect and unhook the ribbon cables. A chance the ribbon cables are loose or have gone bad. Then you could power back on and see if it works on that sat without the module. Also inspect the module for discoloration which could mean it has a problem. If after all that you still are not getting anything then the module needs to be installed or it could have gone bad. One could always check the module with a multimeter as well. Just for fun I disconnected the loop out cable and lost signal on both 20000 and 21500 but I never unhooked the ribbon cables on the inside to see if it made a difference.
If memory serves the SV8000 will work with 20000 without it's module but not completely sure with the NFHD. A bit rusty on some of this.
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