Mac Disappeared From PC Network

First off a network overview:

AT&T Gigapower with the 5268AC Modem.

Various Macbooks, Mac Mini, IPhones, IPads, androids, DVD players, etc connected WiFi

LAN cable to TP-LINK gigabit switch to drive older desktop PC, DirecTV DECCA, etc.

Issue: Had AT&T installed on Halloween day. First off, have lost Network attached storage capabilities. I take it the USB on the modem tower does not work in this way?

Anyhow, big issue that I did not notice until tonight. Normally, under my old Time Warner internet set-up with an ASUS router (only difference in the above configuration), my Mac Mini which is connected to the network via WiFi would show up on the Network screen of the hard-wired PC.

Again, nothing has changed except the addition of the Gigapower and the new modem tower. Tonight, nothing I did could get that Mac to show up. I changed the Workgroup name. I restarted both computers. I changed passwords. I disabled and reenabled SMB and AFP file sharing. I even changed the hard wire of the PC from the switch directly to Port 2 of the AT&T modem.

This Mac will not show up! 

It is important because the old PC contains a VPN gateway to my work, which runs special software. I work on files on my mac and save them to a big 4 TB drive. Then, I would open my Mac Mini and access the 4TB drive from my PC and drag those files into the VPN to my drives at work as necessary. Without this, I am down to saving files from my Mac to dropbox and downloading them on the PC, even though they sit RIGHT NEXT TO EACH OTHER and have talked prior to this.

This whole set-up is now VERY frustrating and I've spent the last 4 hours trying to troubleshoot! Any IT help would be appreciated.

Thank you!