Thanks.
In looking at the Surfboard’s manuals and what appear to be AT&T’s modifications to the firmware,  having it in Bridge mode is exactly what you need to do.   That way,  the WAN address your TP-Link reports is your real WAN address.
I will proceed as if this is correct.  If not, ignore what follows and update me.
Your Archer AC1750 has UPnP (universal plug and play).   Plex will do the port forwarding on its own if enabled in the modem.
Please verify this is your modem.  http://www.manualslib.com/manual/646508/Tp-Link-Archer-C7.html
If so,   slide down to page 89 and you will see where to enable UPnP.
If so far so good,    Log into your router as the admin user and get to it’s main menu.
Go down to Forwarding → UPnP   (as directed to by page 89 of the manual and enable it.
Save the configuration.   Leave that browser tab open in case you need it  
Go back to Plex - Settings - Server - Remove access
Make sure you’re logged in to your plex account and,  verifying you don’t have ‘specify manual port’ checked,  click connect.
This will use the UPnP port forwarding you just enabled.
Plex will connect and return back to you with green indicators and the actual port number
You should be able to go back to the router tab,  refresh, and see it telling you Plex is indeed port forwarded.
If this didn’t go as described.   Make certain the AT&T modem is still in bridge mode and let me know what you got for results… Also, we’ll go peek at the Plex log files and see what it got back as the error code from the your TP-Link router.