Smartphone communication

I was wondering about communication between two android smartphones.
I know probably the best and the most user friendly solution is to have remote server and communicate through it but what I want is a simple and independent smartphone-to-smartphone communication.
Do you reccomend any simple solution to achieve it?

After 1 minute of googling I found this unity game engine - Unity3d WIfi direct (P2P) android support - Stack Overflow which led me to this MNF (Mobile Network Framework) | Network | Unity Asset Store

There’s also this WifiDirect Plugin | Network | Unity Asset Store

But those are paid plugins. There’s also an example project by Unity using NetworkDiscovery here, but I don’t know if it will work on the current version.