It's still a bit confusing
If we are taking about dimwits that have exposed SMB (which was a bad idea in the 1990's FFS...), there are a few issues with the theory as it stands;
By default, Windows will only allow file share access when in private firewall mode.
Consumer grade modems don't have any port forwarding enabled (again, by default) - even that flawed facility, UPnP, didn't tend to dynamically allow it.
A large number of ISP's will automatically block such ports for their subscribers (unless you request them to be open).
Very, very, very badly configured VPN's or shockingly bad gateways seem to be the most likely vector. If you had, or still have a private network with internal DNS, SMB, NMB etc. exposed, then I suggest you change careers voluntarily, before you are lynched...