I have a test application running at test.rsscloud.io on ports 80 and 9876.
The following code works:
curl --location --request POST 'https://brokenriverbooks.com/?rsscloud=notify' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'domain=test.rsscloud.io' \
--data-urlencode 'port=80' \
--data-urlencode 'path=/feedupdated-s8759' \
--data-urlencode 'registerProcedure=' \
--data-urlencode 'protocol=http-post' \
--data-urlencode 'url1=https://brokenriverbooks.com/feed/'
However if you change the port to 9876 it fails. For some reason the rssCloud plugin seems unable to hit ports that are not 80.
The WordPress server follows the rssCloud spec. So since you don't specify the domain in your call, it does a POST to your IP address with the specified port. If you want it to go to feedland.org:80, you'll need to specify the domain name and handle the appropriate validate endpoint where it's a GET request with url and challenge parameters. See http://walkthrough.rsscloud.co/#challengeParameter