I found a way to make L2 and R2 work on Android via Bluetooth
So, I had this problem a few days ago ever since I bought the Stadia controller online. The L2 and R2 buttons don't work on most of the games that I've played. Though the rest of the buttons do. I've learned by researching that the buttons are analog inputs instead of digital, which is the sole reason some games don't recognize it.
I've checked this other forums for the solution, but most of them only apply for PC. So I dug deeper.
I found this apo called "Key Mapper" which allows you to not only remap your phone buttons, but also remap your gamepad inputs! Neat, right? This allowed me to remap the L2 and R2 buttons from analog to digital input. This way, the games will recognize it.
So what I did is turn on the accessibility settings on my phone and make sure that the app runs in background and prevent it from getting killed.
And voila! It worked like a charm. I was able to test this on emulators, Wuthering Waves, Zenless Zone Zero which previously had this issue, but is now working okay.
Hope everyone that's also having the same issue find this helpful!