Find a working RSA implementation source code that can work with arbitrarily large integers, that means integers can have 1000 or more decimal digits. An open source code that you can use is given here

GitHub - abdallahelattar/RSA-and-Big-Numbers-Implementation: RSA Encryption and Decryption using Public and Private keys including other operations. (This code works in Visual Studio (C++ empty project), but there is no clear explanation of functions used. Some large primes having more than 100 digits are provided in the Rea_Me file.)