UPDATE 2: The above workaround does not work for the Roon Arc app on IOS. Unfortunately the login process still hangs and so means Roon Arc is unusable.
I whitelisted ‘roonlabs.com’, ‘roon.com’, ‘roon.app’, and ‘account.roon.app’ in my IOS ad / popup blocker program ‘1Blocker’ and allowed popups in Firefox for IOS but this did not help. Next I turned off the ‘Block Popups’ and ‘Content Blockers’ in ‘Settings > Safari’ on IOS and this seemed to work.
Once I entered my credentials again in the ‘account.roon.app’ page (which was now visible!) then I was authenticated successfully and logged in the the Roon ARC app.