Private servers exist in a legal "gray area." They infringe on the intellectual property of the developers, and the creators of these servers often face "cease and desist" orders. From an ethical standpoint, they bypass the monetization model that keeps the official game’s story and high-quality animations in production. Conclusion
These servers operate by tricking the game client into connecting to a different IP address. Instead of communicating with the official Aniplex servers, the client talks to the private server. fgo private server
There is no functional, public "private server" for the mobile game that allows for free Saint Quartz or custom rates. Private servers exist in a legal "gray area