I'm running a simple NodeJS REST API and a ReactJS frontend application and I want them to communicate in a secure way over TSL. I've successfully issued a certificate (DNS challenge) from Let's Encrypt and I'm currently using it for my IIS site where the React application is running.
I now want to use this certifiacte in order to make communication between my frontend and backend (more) secure.
Both the clients where the frontend will be accessed from and the server where the backend is running are in the same LAN and not publicly accessible. The only thing I want to ensure is that the data is somehow securely transferred from the clients to the server. (You shouldn't be able to see the password and username just by pressing F12 in your browser, neither should a Man-in-the-Midle be able to do so)
I'm pretty much a beginner when it comes to network security/securing communication between client and server. (You've probably already noticed that by now)
Thanks in advance for your help and recommendations.