ᐅ How do you securely mount BILLY shelves in older buildings with crumbling walls?

Hello everyone, I have the following problem: I want to install my Billy shelves in an older apartment where the walls are quite crumbly, with some plaster even falling off. The usual wall anchors don’t seem to provide enough support here. My question is: which method, technique, or material would you recommend to securely and permanently fix these shelves to such fragile walls? Are there special anchors, maybe some kind of reinforcement, or alternative mounting options that are practical for older buildings with these wall conditions? I look forward to any tips, especially if you have practical experience. Thank you!