Same happened to me. Our old Show stopped working. Our Fire TV’s never worked. And then we replaced our old Show with a new Echo Show 21 (technically not supported by monocle) and that had the same issues.
Your post helped me identify the problem. I was struggling trying to get the Monocle gateway to work, because I thought it was just that Amazon was requiring encrypted streams. But it turns out I wasn’t doing anything wrong. Amazon tightened the screws yet again.
Until Monocle has a solution, I turned to Scrypted. As long as you don’t use the cloud recording and alert features it’s free. I installed it as a windows service using a script from their website. It’s a bit more involved than Monocle, and I had to reconnect things a couple of times to get everything refreshed and showing up, but now my cameras work like they used to. As far as I know, Srypted is currently the only free option which uses the Amazon API required. Spent a few days on this so hopefully this will help someone else save some time. Good luck!