Hi Jack - I used to own an FP30 and was able to connect it to an iPad Air running on iOS version 11.4 without issues.
The process was convoluted but after an update (can't remember if it was an iOS update or the Piano Partner 2 app), it became a lot more simple. This process still works now for my Roland HP605 so should not be an issue.
Aplogies if you tried this already but what I found and worked for me is to ensure that the FP30 is running the latest firmware (currently ver.1.05) and can be downloaded here:https://www.roland.com/global/support/by_product/fp-30/updates_drivers/
and the instructions are on their website on how to perform this firmware upgrade.
Then on the iPad (under Settings, Bluetooth, Devices) remove any reference of the FP30 (if any) using the i button next to said reference and select forget.
Then launch, Partner 2 app, select the Settings icon (top right hand corner), press Bluetooth Midi Device and the FP30 should appear in there in a NotConnected status - select it and the app will connect to it.
I never had to use the camera USB dongle because BT worked and was easier than messing around with the cable. Let me know if you need any more info.