Installation of a shower tray, problems with drainage to the sewage system.

Yes — ideally the sewer connection should be lower than the shower tray siphon, because if the discharge line is higher the water will drain slowly and some water will remain in the siphon and pipe [#14149943] If the floor height makes that impossible, you can place the siphon outlet and the drain almost at the same level as a last resort, but keep the run as simple as possible with as few elbows as possible and maintain a slight slope in the 50 mm PVC line [#14149943] For a combined siphon, the upper pipe goes to the overflow and the lower one goes to the tray drain; the flexible connector is there to make the connection easier, and a 40/50 reduction may be needed because the siphon hose is smaller [#14150804] When connecting into the 50 mm pipe, it is enough to insert only the end of the siphon hose into the socket [#14151340]