The summary.
WHAT A PORT IS FOR
One address, many services. The port says which service a connection is for.
A service working on one network and not another is usually a blocked port, not a failed service.
TCP AND UDP ARE DIFFERENT
A firewall rule for one does not cover the other. Check which the service actually uses.
TCP slows down in response to loss; UDP does not, which is why calls degrade audibly while downloads simply stall.
ALWAYS PREFER THE ENCRYPTED VARIANT
HTTPS over HTTP. FTPS or SFTP over plain FTP, which sends credentials readable.
WHAT STATUS CODES TELL YOU
Which side to investigate. A client error and a server error need entirely different responses.
BEFORE EXPOSING ANYTHING TO THE INTERNET
Understand that it is found by scanning within hours.
Never expose administrative interfaces, remote desktop or database services.
For most small businesses, hosting elsewhere is simpler and safer than exposing equipment at the premises.
THE CHECK PEOPLE NEVER MAKE
The clock. A drifted clock causes certificate errors, rejected two-factor codes and authentication failures.
