He signed up April 1915 but went AWOL in October and was declared a deserter, March 1916.
There is a letter in his military file from Lottie's lawyer, Joseph Lynch, in 1941 - which states that she had not seen him since about 1914, that she was seeking a divorce, and they were looking for information about his current whereabouts.
So where was he between the wars, if he didn't go home to Lottie?
In 1941 he seems to give a sister as next of kin. Perhaps he at least got back in touch with his birth family - if "Robinson" was a sister/niece/nephew etc, that would explain why they knew about Lottie and the four kids.