At the end of the previous series (and in flashbacks depicted at the start of the movie) Felix enlisted in the Canadian Navy. In this movie, he's depicted as serving in the Canadian Army.
The movie depicts the events of the first Christmas of the War in December 1914, before which time Felix goes missing in action in battle during an attack. But the first battle of the War in which Canadian land forces were engaged was Neuve Chapelle in March 1915, and the first battle in which Canadian land forces participated in an attack was Second Ypres in April 1915.
Throughout the movie, characters turn on and off electric lamps that are plainly modern-day pushbutton-operated. Electric lamps of the period were turned on and off by turning a key.
The Canadian flags depicted in the movie are the modern Maple Leaf Flag, which was first designed in 1964. The flag flown during the War was the Canadian Red Ensign.