Are you using latest firmware? Can you login web interface when connection drops?

I was confused as you before. I contacted TP-LINK people and now I know what's going on. TD-W8970 with three antennas and two USB ports is V1. The one with two antennas and one USB port is V3....

Don't try to plug WA5210G to CPE510 and your WA5210G may be damaged. As I know, CPE210 is 24V passive PoE devices. As to specifications you are interested in, you should contact TP-LINK directly....

You can try to use WDS mode--four address WDS.

How about using TP-LINK as an access point? http://www.tp-link.com/en/article/?faqid=417

As i said, try to set up a PPPoE connection on your PC directly to the modem and test if it works. You know how to do that, right? Here is an example for Windows 7....