To use this mod, download latest Melon Installer from https://github.com/LavaGang/MelonLoader.Installer/releases/
Use it to install MelonLoader into your game
After that, copy mod .dll file to Mods folder in your game folder
Start the game once, and after that close it using menu

Login into Photon Engine dashboard at https://dashboard.photonengine.com/
Create a new app with Quantum Photon SDK
Copy App ID of created app
Go to UserData folder in your game folder and open MelonPreferences.cfg
Change AppId field to the one you copied earlier

Congrats! You can play online now.
To play with friends they will have to do the same steps. They shouldn't create new Photon app, but instead use the same app id you have.