Sewer Gas Odor Diagnosis & Repair needed

It happens primarily during colder weather and is starting to become noticeable again as temperatures cool down. I’ve already done quite a bit of troubleshooting. The shower and sink traps have water, the toilet has previously been temporarily sealed, and the smell appears to stop when the ceiling fan/heater opening is sealed with plastic. Turning on the heater portion of the fan makes the sewer smell significantly stronger. There are two bathrooms/toilets directly above this area. The ceiling also appears to have been opened/repaired by a previous owner, so there may have been prior work in this area. In very cold weather, the plumbing vent on the roof has also developed significant frost/ice buildup, although I don’t know whether that is related. I’m looking for someone experienced in diagnosing sewer gas / DWV leaks, ideally with the proper equipment such as DWV smoke-testing equipment, who can locate the actual source and repair it. I’m not looking to randomly replace wax rings or other parts to see if the problem goes away. I’d like the source identified first. Opening a targeted section of the ceiling is fine if necessary once the likely leak location is established. Please include in your bid how you would diagnose this and whether you have experience/equipment for DWV smoke testing or similar sewer-gas leak detection.