First off, these speeds are very good.
now… we just got fiber yesterday and the speeds are amazing but I can not seem to get plex to work (or a lot of things for that matter) with the AT&T hardware. From what I understand there is no way for the modem/router to go in to bridge mode, and so I can not seem to get the router to be accessible. I have set a DHCP range, and tried doing static IPs. For the life of me I do not see how to do port forwarding on the AT&T hardware or assign static IPs and you also can not set up DMZ plus mode for any device that has a static IP so when I tried setting my router to its own IP I could not open up the DMZ plus mode. Does anyone here know what to do? I have been very excited for the upload speeds of AT&T for both plex and needing to move large video and data files over my network for work but this apparent inability to do basic things or use your own hardware makes me glad I have not cancelled my Spectrum 300/20 plan.
Without knowing anything about ATT fiber. I would assume that you could get your own router and put behind the ATT appliance, set it up with its own subnet and put its IP as the DMZ in the ATT appliance to avoid double nat’ing. I wouldn’t think the ip should change since it is technically the only device that the ATT appliance would see and as such the IP wouldn’t change. Not sure if this approach will work, but it’s worth a shot.
What model AT&T router do you have? I also have AT&T fiber, have their router/modem passing the public IP to my own router without issue.
Thanks guys, I will be giving it another shot here this weekend and will give and update. My server offline for too long made me too sad and had to throw it back up on the original line for the time being 
My apologies on the horribly long delay and thanks for all your answers. I started having HDD failure which resulted in a new server build (it was time to upgrade capacity anyway) and that rebuild is still happening with the last of my data going over now… but I digress
This whole time I flipped back to TWC/Spectrum to have everything online while i figured out how I was going to be moving forward. with both the server build now and the Spectrum service coming to an end I switched back over to the ATT service and already tonight I am facing the same issues. It seems like they may be Double NAT related but I am having an impossible time figuring out how to get this taken care of. Any help would be GREATLY appreciated.
Network setup currently: (cat6 everywhere)
fiber coming in and connecting to ATT box (model 5268AC FXN)
ATT box connected to Personal Router (Nighthawk R8000)
old and new servers both directly connected to personal router
switch/patch panel to house connected to personal router (the computer I am on)
Network Settings: Port forwarding is set up the same was as it was previously on my personal router, I have changed the port forwarding to match the new internal ip address though. I have also set the “Applications, Pinholes, and DMZ” on the ATT box to DMZ Plus mode for my personal router
Plex opens up and works just fine on my local network, and is saying that my “remote access is enabled through a manually-specified port” it works but on my phone it has an exclamation mark in an orange circle next to the server name. I have even tried putting the R8000 in bridge mode to combat the possible double-nat. The fact I can not put this ATT box in bridge mode is really annoying me. If anyone has any experience or advice I would be so so appreciative.
One other thing worth noting, my plexpy is saying it is experiencing an error when trying to communicate with my server and is not detecting anything.
Review This link for ensuring you have the DMZ+ setup for AT&T router properly to forwarding the external IP to your internal router. I once had that model of router and got it working, but changed to the NVG599 as it was more stable and could handle traffic better.
