TCP port 445

Do you mean you can't reach your iMac from outside of the local network?

If so, and your new modem is also the router, you would need to poke a hole in it with port forwarding so the rest of the internet can get to the server, too.

If you are going the other way and trying to get to a server outside of your local network, then did the new modem come with any kind of security software that might be blocking connections to the outside?

sandrine40 wrote:

I can't reach anymore a server by using smb link on my iMac OS Ventura (13.4.1) since I received a new modem... what should I do?

See if there is anything here—

Set the protocol for file sharing on Mac - Apple Support

That port 445 is associated with a Microsoft domain server and I would ask the administrator if they changed that to another port, or are restricting access on that 445 port.

Is the server yours? If so, then maybe look at using TailScale.com so that all the systems you control look like they are on the same local LAN, even if they are not at home.

If this is someone else's server, then you need to figure out what the new router is doing to your network connections.

For reference bookmark the Apple Support Document TCP and UDP ports used by Apple software products - Apple Support!