are you talking about the DJI Focus Handwheel 2 or the DJI Focus? If it's the former, once its connected into your controller via the DJI CAN bus cable, an icon appears along the top menu bar in the DJI Go 4 software that looks like the DJI Focus, click on it and enable the device. Then it just works.
We have solely been setting up the Handwheel and Handwheel 2 on I2s and have not worked with the regular “DJI Focus” on the I2 at all. I can’t say that I’ve seen anyone running the standard focus unit on I2. Do you have an X5S to see if it works on that? I will ask around at our office tomorrow if anyone has seen any operators running the Focus and I2 to see if we can provide any insight. Understand the desire if the Focus is a sunk cost that you likely use on a Ronin but IMHO the Handwheel is much better for aerial/RC mounted.