Sound insulation toilet cistern

Who gave the tip? Was it the plumber?
There are "better" versions of waste and drainage water systems that do not transmit noise as strongly. For wastewater, this includes, for example, thicker wall thicknesses of the pipes. When refilling, it is more a water rushing sound (flow noise) that disturbs. Perhaps adjusting the dimensioning of the supply line would help? Presumably there are extra quiet flush tanks (possible to test listening? :D)
Otherwise, a lot of weight promotes sound insulation, which the wall currently lacks. Attach/stick bitumen or rubber mats? By the way, we use construction protection mats for this (coarse rubber granulate pressed into mats), cut to size to acoustically decouple installations. Pre-wall elements are therefore placed on rubber and then screwed, exactly the same at the wall.

11-layer Poroton. What else is there to say.

EPS is nonsense, it has no mass. You want sound insulation, not thermal insulation ;-)
If anything, then rather mineral wool or proper soundproofing materials.
Ask the plumber which products he can use to prevent the problem.

Otherwise, at least don’t flush pee at night ;-)

Who gave the tip? Was it the plumber?
There are "better" versions of drain and supply water systems that do not transmit noises as strongly. For wastewater, for example, this means thicker pipe walls. When refilling, it is more of a water rushing sound (flow noise) that disturbs. Maybe adjusting the sizing of the supply line helps? There are probably extra quiet cisterns (test listening possible? :D)
Otherwise, heavy weight usually promotes soundproofing, which the wall currently lacks. Apply/stick bitumen or rubber mats? By the way, we use construction protection mats (coarse rubber granulate pressed into mats) cut to size to acoustically decouple installations. Pre-wall elements are placed on rubber and then screwed in, exactly the same at the wall.

No, not the plumber (at least not the one doing the installation), but someone with experience in the plumbing field. He told me that the noises in new installations are not comparable to old installations, and as you also mentioned, the refilling noise can be the most disturbing.

11er Poroton. What else can you say about that.

As said, that’s not what this is about. The conditions are now given as is.

If you really want to achieve something now, then only cladding with drywall or something similar remains. Maybe even additional insulation...

We have an 11 Poroton between the kitchen and guest bathroom..... I find the refilling noise annoying because water keeps running afterwards. It's now towards the kitchen but not bad. What’s also annoying is the flushing and especially what I will call the trickling sound.... with the toilet itself, make sure that the jet does not go directly into the water (no joke), there are different models... I don’t have a solution now but I definitely want to advise you to find one, because if you sleep next to it, it’s really annoying!