How to get rid of echo inside the apartment?
Probably as a result of opening the wall to repair the plumbing, the wall facing was rebuilt.

You might try texturing the wall and repainting it with a non-glossy paint.  Just matching the other walls should be good enough.

Also, if you have not yet returned all wall coverings, tapestries, paintings, and furniture to that wall or carpet to the floor, the room will sound much "brighter".