Maybe your monitor don’t support for 480p. It don’t work if you force to change the resolution to 480P.
Also you have a try following below steps.
2. Replace TvSettings application with WI-FI adb command.
# adb connect IP_ADDR
# adb root
# adb remount
# adb push TvSettings/TvSettings.apk /system/priv-app/TvSettings/
# adb push TvSettings/oat/arm64/TvSettings.odex /system/priv-app/TvSettings/oat/arm64
# adb shell sync
# adb shell reboot
After do that, you can see the 480P resolution on TvSettings application list.