If you try to POST to a static page, Nginx will return a 405. That's okay; it's expected.
To solve the issue, you must ensure WP-Rocket does not create a static page for your custom login page. I don't know how you do it, but a configuration must be somewhere.
So, it's not a WordOps problem (and not WP-Rocket, either, I guess). There's nothing for WO to solve.