Placing a favicon in the root phpBB folder worked with Chrome and Firefox but did not work on iPads or other mobile devices. Favicons were working on a second website so I used the same configuration with phpBB and found it works great. In addition to a favicon you also need a browserconfig.xml and manifest.json file. They should be placed in the phpBB root folder. Then add the declaration codes to the overall_header.html file for your template. Place the code just before the closing </head> tag.
Instructions and a download link, for the files, are available here