I sent out an email after 48 hours of dealing with trying to open up my ports for Bitcoin, I was quite frustrated and angry since I had to call like 10 times and I was making zero progress. Most of the AT&T people didn't give me any helpful clues on how to fix the situation. The original email described how there is a firewall in the DVR, and I thought it was blocking the ports. It is true there is a uncontrollable firewall in the DVR, it is false this blocks 8333.
The actual problem is due to AT&T Uverse customers being forced to use a private dynamic IP, the IP is literally hidden from the internet, so it isn't possible to send any requests at it. It will literally ignore pings across all ports. So the solution is to switch to public static IP and make sure you allow incoming traffic.
It's not so simple though, AT&T will not let you have a public static IP without paying. I've had my router reset 10 times today by AT&T (probably automatically) and it comes back with a private dynamic IP. Then I have to reset it to use public IP and that lasts less than an hour. It literally went from open to closed while typing this email... the IP address went from public to private dynamic.
https://i.gyazo.com/3c732687fc3d21acb7d62f6d0e23a346.png
This is making using Bitcoin Core almost impossible. I'm at least getting some synch now but maybe a few days of blocks the entire day, cause I can't sit here all day with the computer and keep fixing it.
The proof is in the pudding, there are 37 nodes using AT&T in the ENTIRE world. AT&T is a massive ISP so this is strong evidence that using Bitcoin Core as a full node on AT&T is extremely difficult and actually just about impossible.
https://i.gyazo.com/90beebe056f5fc338165e8d200536c06.png
The other big ISPs have pathetic numbers also due to the same sort've things that AT&T does, but at least Comcast has 400 nodes. AT&T is much harder to use than any other ISP I've dealt with when it comes to Bitcoin Core.
I apologize for sending out the wrong info the first time, although it is still worth noting the DVR firewall is out of your control, which might be a problem if not now then in the future. In any case AT&T has effectively blocked full nodes for Bitcoin Core via the private subnet, and the disability to change it to public without paying $15 more per month, and buying a $15 connection service so they will give you that info (if you dont pay the connection 'specialists' hang up on you).
It is important to note this is not Bitcoin specific, but effects every program that depends on freely open ports. I don't think AT&T has anything against Bitcoin, it's just their security settings and policies have disabled Bitcoin Core for most customers. Also important to note this isn't a problem specific to AT&T, all the big ISPs are doing similar things. I believe the changes in ISP protocol are the main driving force behind the massive decline in Bitcoin nodes. Another big factor is firewalls, most people can't even remove the firewalls enough to open ports at will. The community needs to educate people on how to use Bitcoin Core when facing these intensifying security measures, or the decline of node numbers will continue.