There is a way to search for comments using the "global search", but no way to search for text over issues and their comments. In particular, no way to search from the issue tab, no way to search over comments only in issues (or only in merge requests), no way to combine a text search with label/milestone/status filters, etc.
So it's a workaround, but a bad one.
Here's the ticket (2015): https://gitlab.com/gitlab-org/gitlab/-/issues/13891. The fact that it has so many duplicates in your own project's issue tracker is a good indicator of how bad your issue search is.