Set RenderTexture to camera in runtime.
You can use Package Manager or import it directly.
https://github.com/XJINE/Unity_RenderTextureSetter.git?path=Assets/Packages/RenderTextureSetter
This project use following resources.
Add RenderTextureSetter.cs
into your camera object and set the texture size and format from Inspector.
When set 0 into the size, it will be Screen.width/height.