If you're still having issues with this and need assistance, first give us a bit more information on the matter:
• In the title you mentioned changing both IP and FQDN. Are you indeed changing the FQDN as well?
• Since you mention modifying the DNS record I take it you're using a custom FQDN instead of a 3CX one can you confirm?
• What do you mean "nothing works"? Are you unable to reach the 3CX Management Console with the FQDN?
• If yes, have you tried reaching the 3CX Management Console using the new public IP?
A few things to check and try:
1. Make sure the new PBX has all
required ports forwarded to it.
2. Confirm you can reach the Management Console when using the new public IP.
3. Confirm the FQDN resolves to the new IP and you can therefore reach the Management Console with the FQDN too.
4. Make sure that the actual remote location (where your IP Phones and clients are) can actually resolve the FQDN to the correct IP too.
5. Make sure you've shutdown the old machine and that the new one can activate successfully. On the new machine go to "Settings >> License >> click on 'Refresh License Key Information'"