WA830RE is working as an access point or range extender? What's the firmware version of your WA830RE? Can you describe the problem in details?

Unfortunately, it has no access point mode, range extender mode ONLY.

W8980 is the main Wi-Fi router? I don't think that you have to enable WDS on it. WA850RE can be configured to extend Wi-Fi range. But none of TP-LINK products support seamless wifi roaming actually,...

Any update there? :)

Check if your TD-W8970 is running in latest firmware. If not, upgrade it first, reset and reconfigure it to test. http://www.tp-link.com/en/download/TD-W8968_V3.html#Firmware With the latest...

Both TD-W8970 and TD-8816 are ADSL2+ modem routers. If TD-8816 works with your line, TD-W8970 should be OK as well. How did you test? Did you try to change DSL Modulation Type on its web interface?...