I'm reviewing a paper on a theoretical topic in the physical sciences. My opinion is that one part of the paper is valuable and deserves to be published, but another part needs attention from a reviewer with expertise that I don't have. (It wasn't obvious when accepting the review that this would be the case, otherwise I would have declined to review it.) I feel that that part of the paper should be reviewed by someone with the relevant expertise before publication.
I said as much in my initial review. However, the paper has now come back to me along with the other reviewers' reports, and none of them commented on the part that I wasn't able to review.
This puts me in a funny position, because if I recommend acceptance there is a possibility that it will be published without anyone checking the relevant section, but if I recommend reject then I'm recommending rejection purely on the basis that I personally didn't have the expertise to review part of the paper, which doesn't seem fair. I could explicitly state that it shouldn't be published without another review, but that seems like I would be overstepping my responsibility as a reviewer.
What is the right thing to do here? Should I contact the editor, or am I just over-thinking this?