Afaik Joyce is usually listed in the first dozen when talking about modernist writers. For myself, I explain this as: modernism being sorta the culmination of linear progress in laying out narratives, and literature's attempt at self-analysis in the context of this grand tradition with picking out techniques that are useful and ones that are decorative; the last phase before 20th century's all-out deconstruction and nonlinear mindgames. Joyce IMO utilized pretty much every technique he could think of and juggle, in ‘Ulysses’. Though frankly, by the same token ‘Ulysses’ could be just as well considered among post-WW2 postmodernism, so dunno.
I could be totally wrong, however, and led astray by architecture, graphic and industrial design where ‘modernism’ is rather straightforward.