Not able to find Lennox ML193UH furnace filter
If you have a filter in the return air grille, that's likely the only one in this system. Nobody wants to climb into their attic just to replace an HVAC filter, so when a furnace or air handler is installed in the attic these filter grilles allow placement of the filter in a more accessible location. In fact, according to page 6 of this spec sheet for that model furnace, a filter rack is not built into this model and is available as an optional accessory.

So unfortunately, something else is wrong with your furnace...