@griceluke I had the same issue with Monocle unfortunately. I passed the camera feeds through Frigate and used the restreamed feed from there to feed through to Alexa rather than direct from camera.
Otherwise, you could try using ONVIF Device Manager to absolutely confirm the url and if you have 2 streams or more available, start by trying to view the lowest res stream first on the Alexa device.
I did run into a camera that I could not get to work in Monovle no matter what I did. I ended up after WAY to many hours of trying to solve it, moving to Scrypted. That also works with Alexa. Good luck!