What would you want to call them?
First, realize I didn't comment on "them", but rather suggested the term "anti-Trump Republicans" was an oxymoron
Nonetheless, to answer your question, in modern terms, of course, they are referred to as RINO's. That will do for public discussions although I think the label much too generous.
Since Trump (R) became President and de facto leader of the R party, "anti-Trump" describes an actor as one that publicly or otherwise works against Trump and his strategy (as opposed to tactics). Under what theory can they be regarded as R's?
Does their self-affixed label make it so?
Have a nice weekend.