I too have the AT&T 5268AC router, this the steps that tablo send me last year hope it will solve your problem too.
Hi there,
Thanks for contacting Tablo support.
The following steps should allow you to get port forwarding set up so you can use Tablo connect.
You will need to log into your router’s settings by entering the default gateway into your browser’s address bar.
The login information is typically on a sticker somewhere on you router. If not you may need to contact your Internet Search Provider.
Once logged in, go to “Settings” then to “Firewall”.
On the Firewall page click “Applications, Pinholes and DMZ”.
In the box marked “Enter IP address” type in your tablo IP address
Choose “Allow Individual Applications”.
At the bottom of the page, click “Add a new user-defined application”
Type in “Tablo” as the profile name then check the TCP item.
The port range should be 21140 to 21140
The “Protocol Timeout” can be left blank.
In the “Map to port host field” should be 80.
Click “Add to list”.
The entries for the first rule clear allowing you to enter another rule.
Check the TCP item.
The port range should be 21141 to 21141
The “Protocol Timeout” can be left blank.
In the “Map to port host field” should be 8887.
Click "Add to list"and click “back”
In the box marked “Enter IP address” type in 192.168.1.67 again.
Choose “Allow Individual Applications”.
Then choose “Tablo” from the application list and hit “Add”.
Afterwards scroll down to the bottom of the page and select “Save”.
Once saved, navigate to the Tablo Connect section in settings of the Tablo app and hit “Re-test port mappings”
If the ports are not being blocked by any other device, the Tablo Connect feature should read “Your Tablo is ready for remote streaming.”