Slow sink
The grease cap has grown past working volume and the inlet is pushing water into fat. Stop sending water down, do not pour boiling water or any product in, lift the lid and look: grease at the outlet baffle means the tank is full; a thin cap with a dead sink means the blockage is past the tank. Call and say which.
Smell
Settled solids rotting in the bottom compartment produce gas that exits through floor drains and lid seals — worst in the high-desert morning when the kitchen has been shut and still. Bleach does nothing to the source; enzymes are banned and would only move the grease downstream. Run water into dry floor drains, check the lid gasket, then schedule a full pump-out, which removes the solids that make the gas.
Why this is a compliance event
Reno’s rule is that an interceptor “must be cleaned whenever the combined thickness of the floating greases and settled solids is greater than 25 percent of the design hydraulic depth.” A tank that is backing up a sink passed 25% a while ago. Your EC permit interval was either too long for the kitchen or not kept; the calculator shows which.