The ISP does not configure port forwarding.
Once they assign you the static IP it should show up in the Orbi instead of the 100.74.x.x address.
You can then enable port forwarding in the Orbi using either UPnP or manual port forwarding.
UPnP is automatic. You enable it in the Orbi (it may be enabled by default). Plex Media Server then negotiates with the Orbi to open the port. On the Plex remote access page you do not check the “manually specify public port…” box. The port number will change every time remote access is enabled, PMS is restarted, etc. This is part of the UPnP protocol and is OK.
With manual port forwarding you configure the Orbi to use a fixed port number that never changes. You also configure the same port number in Plex Media Server and check the “manually specify public port” box. Plex always uses the same port number. Manually specified ports are useful with routers that do not support UPnP or have trouble negotiating with Plex to open ports.
Read through the Remote Access and Troubleshooting Remote Access support pages. The Basics of Remote Access Troubleshooting also contains helpful information.
Summary:
Step 1: Get the static IP.
Step 2: Make sure the Orbi sees the static IP.
Step 3: Configure port forwarding in the Orbi, either UPnP or manual.
Step 4: Configure Plex Remote Access.
Reply back with progress, issues, questions, etc.
