Shower sewer doesnt evacuate water correctly

Hello guys,

My shower sewer doesnt seem to be working properly, Its so slow that if I spend more than a minute with the water on the plate starts flooding. I haven't seem this installation before, It consist of a regular strainer (we added a net to prevent hairs going down, but it does not affect the draining rate) and underneath there is a cube of approximately 7cm by side (2,7inch), and the waters drains via an open in an upper edge on one side which height is around 0.5cm (0.2 inch). I tried unclogging it with vinegar and baking soda but the mix happens in the cube, therefore not affecting the draining system whatever it is. I dont know what can I do.

If you can point me any useful tip I'll appreciated.

PS: I cant pass beyond the edge so I dont know what is in there, just that my loft used to be a garage, and the electrical installation is a botch so I dont doubt that plumbing is the same.

https://preview.redd.it/g30unxyuyfpe1.jpg?width=1920&format=pjpg&auto=webp&s=d7c9e9de673eb89ce5c33523e1e69d174a58923d