I've been searching for this series for a while, the story revolved about a group of guardians of sort that travelled between parallel dimensions, through specifically hidden gateways in each dimension, that only said guardians know how to use. None of the people of the parallel dimensions knows about other dimensions, only said, guardians. As I recall, initially the main character often travels with his uncle. I am unsure about the date of publishing of the book, however, I read it in 2014-15.
There should be more than 5 books in this series.