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.
BTW you can see how long it's been since the last cloud renew by going to the Feed Info page for the feed.
http://feedland.org/?feedurl=https%3A%2F%2Frsscloud4.wordpress.com%2Ffeed%2F
In this case it was successfully renewed 13 hours ago.