I'd imagine that if you updated the iOS client to now use the http://joplin.home address it would work since you've granted the app access to the local LAN network. Just a guess. The reasoning behind this would then mean if you had to move the instance to a new IP address, you could do so without needing to update the config on any of the devices.
So the process would be: give Joplin on iOS the 192.x.x.x address once, just to get it to prompt for LAN access, then regardless if that would work or not, change it to the local DNS name you have assigned for it.