Update: During my previous post - I made a mistatement about Monocle. I said there are 2 ways to run it - and this was true but I described it incorrectly. The simple, basic way to get Monocle to run is this: Use the MonocleCam.com website, setup an account then setup cameras. These cameras - lets call them virtual - you can either point directly at the camera which gives the fastest response, or point at the
BlueIris stream. I won't detail the string difference here. The BI stream is MUCH more compatible and I am able to get 5 different manufacturer cameras to display on my Alexa Show's

. Then, if you're still having problems, you can push further and install a local service called Monocle-gateway. This tool is like an internal interface so Alexa can get a) a tls stream, b) a validated SSL response. There are configuration options on the gateway that can be used to get even really stubborn cameras to work.
Some quick info - If I get a direct stream from an ONVIF via the Monocle Web - from Alexa "OK" to an image is about 6 seconds. If I display the stream from the BlueIris server it is more like 11 secs. If I display the BI stream via Monocle WITH a local gateway the time is nearly 20 secs (and the video is actually 20 seconds behind as well!).
I am able to get all my cameras to work WITHOUT the internal gateway. HOWEVER. Recently I implemented a Alexa Show 10" gen 3. Thats the motorized screen model. It has failed. I have done everything I can to resolve this - including trying @proxy, @tunnel, @noaudio, @fakefmtp. I validated the SSL, checked all my settings and I can see the feeds on ALL my Alexas's EXCEPT the new 10" gen 3. I contacted Amazon and got diddly for help. They simply point at the camera manufacturer. Since Onvif is a standard, it's BS. They know it, and I know it. So it is going back. If anyone cares, I have some posts on Monocle gateway including logs from the amazon side. I came here today in hopes of finding notes or progress from BI changes that might improve the situation but see no joy.