Yes you will lose all your data after the install.
You have to install the update_Mygica_ATV1900AC_5.1_NoNetflixHD_20170603.img first.
Then install the update_Mygica_ATV1900AC_5.1_NoNetflixHD_ondemand_patch_20170605.img"
Then you can install the latest patch MyGica_1900AC/1800/582/380_Android_5.1_20170612.img this should fix your Ethernet issues.
With Android 5.1.1 you must turn of the Ethernet to use the wifi or turn of the wifi to use the Ethernet, they don't work at the same time in Android 5.1.1 like it use to in android 4.4.
Hope this helps you.