If you are using an iframe, it is there browser that needs to see both. openHAB is just providing the url.
I never really got the webview to work the way I wanted it to so I use Grafana Image Renderer. To use a webview there are a couple of ways presented in InfluxDB+Grafana persistence and graphing. But in my experience webviews in general don’t work in the Android app, unless something had changed.
If you are using net=host for openHAB and port mapped grafana’s port, openHAB can see Grafana just fine.