Maintenance technicians reported AC duct improperly installed causing air turn back.

Narrative

Repair Station X CRS (Certified Repair Station) is preforming maintenance and inspections on operator based out of ZZZ with little to no experience on make and model of aircraft the operator operates. I have attempted to provide training and guidance with no action take by the CRS. The incident of the air turn back was an example of the technician and the inspector unfamiliar with the air-conditioning ducting resulting in the connection coming lose in flight resulting in an underfloor overheat and smoke entering the cabin. It was discovered by the flight crew. Maintenance found the error upon return of the aircraft and floorboards opened. The ducts were removed inspected for overheating damage and seals replaced.The root cause is the technician was unfamiliar with the design of the system and had no guidance in the task. The Inspector was unfamiliar as well and the proper leak and ops check were not complied with. I do not feel that this is an isolated incident and if the CRS in question does not add or train its personal the potential for addition mistakes in high.