Remote Access failing with setup of ISP Modem bgw320>Deco Mesh Main >Deco Mesh 2nd Point

Plex Media Server Remote Access
1

Server Version#: 1.42.1.10060

I have gotten the Double NAT message over and over. To remedy this I have already tried the following with no success:

  1. From the ISP Gateway, set the ATT bgw320-500 into IP passthrough mode with a fixed DHCPS and the MAC address set to the Main deco mesh (not the deco that my pc i.e. the plex server is connected to; although I did try this address as well briefly without success).
  2. From the ISP Gateway, disable the WIFI signal coming from the BGW320
  3. From the Deco router, reserved the IP address for the server PC
  4. From the Deco router, set a port-forwarding rule for Plex with an internal and external port at 32400
  5. From the Plex settings, both automatically and manually forward the port to 32400
  6. From my server PC, completely shut off the firewall

I’m unsure where the Double NAT issue is arising from considering I thought the issue would be resolved when the gateway was put into IP Passthrough mode. Could this be an issue caused by the TP Link Deco Mesh, since my server PC isn’t connected to the main point?
Any help or ideas to remedy this would be greatly appreciated!