I'm going to take a stab at this one. See if the MIDI Mongoose is sending CC#32 to the H9. That message is commonly used for MIDI Bank Select (LSB). CC#32 is the default MIDI binding for either Input or Output Level. You can change that to something else in the H9 … it's easier in the H9 Control app. I'm thinking that a CC#32 value of 0 may have persisted through your power recycling, giving you nothing to hear at the output.
Mr. Brock may be right. To quote the MIDI spec, "Controller numbers 32 through 63 are reserved for optional use as the LSB (Least Significant Byte) when higher resolution is required and correspond to 0 through 31 respectively"
This means that (say) CC32 will be a very minor change to the value of CC0, and will not act as a normal controller.
This is standard MIDI behavior, and cannot be defeated by changing settings on the H9.