Broken Routing/Loop Issue

I have a GS728TPPv2 that I added to the Insight Cloud Portal. We are currently building a new network in our building while keeping the old network setup active. I put the ISP line into a dummy switch that had a line going to the old network setup to keep the building tenants live and a line to the new setup so I can work on configuring the new network switches. I was working on adding vlans to the ports so that some only had the external traffic straight from the ISP and some ports only had network traffic for an internal trusted network that comes from a watchguard firewall. Everything was working great until I made port 1 (where the isp comes into the switch) a tagged port with the external vlan (999) and management vlan (1). The moment I did that, the switch lost connection to the cloud AND it "broke" the internet for all tenants on the old network set up. See the attached network diagram for more details. Now I wiped the GS728TPPv2 and removed it from the cloud. Even from factory default settings, the moment I connect the GS728TPPv2 to the dummy switch, it breaks the internet for the users on the old network. How do I fix this loop/routing issue?