How to deal with this type of rejection without review report?
Educated guess of what happened: they invited reviewers (which automatically updates the status even if the reviewers don't agree to review). Some/many of the reviewers then declined to review, giving reasons that made the editor decide to desk reject your paper.

I'm not a mathematician, but from what I've heard, 4.5 months is not a particularly long wait for mathematics papers. If so, it's not really fair to say the journal treated your manuscript in a subpar manner.

There's nothing you can do about this except submit to a lower-tier journal.