The link above mentions that he was trained in architecture, but shifted his attention to painting. From the evidence of a photo of him in uniform in the link along with a painting (see below), Robertson served in some capacity in the Great War. Finally, it seems that he was a heavy drinker, this affecting his peculiar marriage arrangement and quite likely his artistic career.
So why am I bothering to write about Robertson? Because he is one of those painters who flipped back and forth between traditional painting and various degrees of modernism -- sometimes even working those styles at around the same time. Moreover, I find many of his images appealing. Others seem to be of the same mind because, even though there is essentially no biographical information, the Internet has a fair number of images of his paintings.