Thanks for the help!!
Was a Factory Reset performed before and after any firmware updates then set up from scratch?
>> I didn't perform a factory reset or any firmware updates. Since it was on the newest version, I left as is.
Internet Service Provider and Modem Configurations
If this modem has a built in router, it's best to bridge the modem. Having 2 routers on the same line can cause connection problems: Link>Double NAT and How NAT Works. To tell if the modem is bridged or not, look at the routers web page, Status/Device Info/Wan Section, if there is a 192.168.0.# address in the WAN IP address field, then the modem is not bridged. If the modem can't be bridged then see if the modem has a DMZ option and input the IP address the router gets from the modem and put that into the modems DMZ.
>> I did already configure the modem for bridged mode, as per AT&T instructions. PPPoE is configured on the D-Link with the correct username and password.
Check ISP MTU requirements, Cable is usually 1500, DSL is around 1492 down to 1472. Call the ISP and ask. Link>Checking MTU Values
>> I did already knock it down to 1492, but will confirm with AT&T if it should be any lower.
For DSL/PPPoE connections on the router, ensure that "Always ON" option is enabled.
>> I do have this configured as well.
Router and Wired Configurations
Some things to try: - Log into the routers web page at 192.168.0.1. Use IE, Opera or FF to manage the router. Besure to log into the Admin account on the router.
Turn off Advanced DNS Services if you have this option under Setup/Internet/Manual or under Setup/PARENTAL CONTROL/Set to>None: Static IP or Obtain Automatically From ISP.
>> I did have this UNchecked in scenarios 1 & 2 I listed, and will UNcheck it again.
Enable Use Unicasting (compatibility for some ISP DHCP Servers) under Setup/Internet/Manual.
>> I actually don't see this as an option to configure, in Setup>Internet>Manual!
Turn OFF DNS Relay under Setup/Networking. Link>Finding Faster DNS Addresses using Name Bench and input new DNS addresses under Setup/Internet/Manual.
>> I had DNS Relay ON, so I just UNchecked it.
Setup DHCP reserved IP addresses for all devices ON the router. Setup/Networking. This ensures each devices gets its own IP address when turned on and connected, eliminates IP address conflicts and helps in troubleshooting and maintain consistency for applications that need to connect as well as mapped drives.
>> Will do this as well, although we don't rely on mapped drives and we only have a handful of devices.