There’s not much to learn from the old post(s) where I talk about that (so I saw no point in digging those up), but in (old question) Manually downloading (installing) apps I e.g. write
I’m using very restrictive firewall rules on the server I have nextcloud running on, that also blocks most outgoing HTTP(S), and I’m not likely to change that.
Incoming traffic to port 80 and 443 are allowed (and have been longer than I’ve run nextcloud, I also have other webthings running), and if it wasn’t I wouldn’t have been able to access the “Basic settings” and see the pointless warning. I guess the key in the above quote is
blocks most outgoing HTTP(S)
because that’s basically the only thing nextcloud can use to test the servers internet connection.
And the comments to your firewall rules are bad. An example: the rule for TCP ports 20+21 mentioned sftp, but SFTP (by default) uses the same port as SSH, which by default is 22, and you have another rule mentioning ssh as the reason for opening port 22.