Ok, the netstat command shows something is listening, that's a good first step.
next I'd make sure it's the mssql:
lsof -i4:1433 will show the process (the i4 means IPv4, do -i6 if you're using IPV6)
You could always try turning off firewalld to rule that out. What I meant about trying to connect from the same host that you are running MSSQL server on: If that works, it tells you that there is a problem with firewall rules.
"I meant that it seemed that the port isn't listening... However, it's been listening ever since I added it to firewalld."
How did you determine this? By trying to ping the port, connect to it using a MSSQL client? Portscan?
If you turn off firewalld, and you try connecting with your MSSQL client and it works, the problem lies somewhere with firewall rules.
If you try that and it doesn't work, I'd turn up logging on MSSQL server and client to see if that gives you any clues. Could simply be a misconfiguration somewhere.