Problems with walk-in shower

Hello! We have only been living in the new prefabricated house for a few months and already have problems with the water drainage in the shower. At first everything was fine until the plumber showed us how to clean the insert in the drain. After reinserting the part, the shower suddenly did not drain anymore. Then a ventilation part was installed downstairs in the utility room, which did not help. After installing a larger ventilation part, it worked at first. Meanwhile, there are problems: when the washing machine runs, the water in the guest WC foams up, when showering it gurgles loudly, and meanwhile the shower is not draining properly again as it was before installing the part. It only drains when the water is turned off. The bathroom is upstairs, the utility room and guest WC are on the ground floor. The company now thinks that something must be clogged and that removing the shower insert was just a coincidence. He now wants to pour in chemical cleaner and says then everything will be fine. Another opinion was that the ventilation part is now too large and therefore too much air gets into the water pipes. We are desperate and don’t know what or whom to believe. Is there someone who can help us, or who has had similar experiences with a walk-in shower? The drain is in the middle of the shower, not as a drain channel. I would like to thank you very much in advance for your help. Best regards sks