The issue turned out to be entering the Host IP address incorrectly. There shouldn’t be anything other than numbers or dots in the address.
Here’s a screenshot from an iPad with the incorrect Host address.
Here it is with the corrected Host address.
Here is what it should look like on an iPhone
Finally, here is where you find the Host address for your computer in REAPER along with all the other OSC settings. The numbers in the screenshots are NOT the numbers for YOUR computer and iDevice.
Still not working? Leave a comment below or on the forum.
Thanks to rsmus7 for the iPad screenshots.