Yeah, YunoHost explains why http://localhost:8536/ wouldn't be working. If cloudflared and Lemmy are in separate containers you have to put an actual IP in, since localhost points to the container itself.

Cloudflare and yunohost user here. Need to install the cloudflared service and make sure it runs and the tunnel connects in the zero trust dashboard. Will say healthy when it does. Try a lower port number, something in the 2000-3000 range is good to test. Then in the correct tunnel once it is showing healthy you need to add a public hostname (for ease of use). Say lemmy.mydomain.com, use https then localhost:portnumber/ Then in additional settings you have to turn on notlsverify else it won't work. That should get it working. To be honest I've not bothered setting up various ports other than the defaults so you shouldn't need to set up the port so just try https and localhost and that should do the trick - this could be where you are going wrong as yunohost doesn't explicity need the port setting in most common configurations. Then in the yunohost admin area you can go into applications and open the app and it should work. Any probs give us a shout

You're running these commands on the same machine as yunohost, right, not from another computer?