If you are on a 1.14 or later you can go to https://app.plex.tv/web start a live tv session and click on the 3 dots to give you signal strength. Another way would be to use a browser and go to the IP address of the Duo and when you tune a channel check the summary for signal strength.
AFAIK, Apple no longer includes the MPEG2 codec so Plex Server is probably transcoding to h.264 with the browser doing a better job than the IOS app. Is this the new UNO client for IOS?