Over here, epoxy would work :-)
All my walls are brick and mortar, they same in almost every house or flat in this country. Ok, some have stone and mortar. And then, the showers have ceramic tiles attached strongly to the wall.
The concept of "studs" is foreign to us. I have been over the pond, so I happen to know about them :-)
Suction cups work initially. The problem is that they get weaker every day till they fall on their own. Or worse, they fall when you are putting your weight on them. You do not know when they are going to fail on you.
I have seen shower enclosures sold on specialized shops that seem a single more or less solid structure, already with grabs installed. Probably the whole thing is glued or bolted to the walls. Surely they sell similar things suitable for stud and panel walls.