WordPress RSS Feed has word limitation.
In stead of full contents, three dots and two brackets […] are shown at the end.
Seems that WordPress RSS Feed full contents cannot be shown.

Someone recommend to choose “Full text” in Settings > Reading > For each article in a feed, show.
In my case, it still does not work.

 

Finally, it is found that the word limitation is set as 55 by default.

wp-includes/formatting.php

$excerpt_length = apply_filters( 'excerpt_length', 55 );

 

You can change the value to larger in order to display more words.
But the change would be fallback after upgrading WordPress.

 

Another solution is to add the following codes in the files functions.php in your theme.

function custom_excerpt_length( $length ) {
    return 500; // set the word limitation as 500
}
add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );

 

Similarly, the change should be fallback after upgrading the theme.

 

So you might consider to install other RSS Feed plugins or request the theme developers to add the feature to define the number of the word limitation.