The manual is quite clear.

https://resolume.com/support/en/connect-grandma2

If you have connected Resolume and grandma2, you can try output few dimmers with a dimmer effect from grandMA2 and check whether there is input in DMX chart in the settings of Arena. You should be able to see signal in the chart just like your dimmer effect.
If everything goes well so far, you can start DMX mapping in Arena.

https://help.malighting.com/grandMA2/en/help/key_remote_control_web_remote.html

You’ll need a good WiFi AP based on the size of your venue. Nonetheless, probably not a good idea if you are trying to go cue in an actual show because of the possible delay.

https://help.malighting.com/grandMA2/en/help/ma_3d/key_videoplayer.html