Wired connection is working OK? Did you try different channels and using channel width 20MHz? SInce you had many TP-LINK routers which had the same problem, did you ever contact TP-LINK people to ask...

@Ddan, did you upgrade the latest firmware here, TD-W8970_V1_140613. http://www.tp-link.com/en/support/download/?model=TD-W8970&version=V1#tbl_j

How does it drop? No ADSL light? No Internet light? No Wi-Fi? Can you login its web interface at that time. If you can, I suggest reporting it here with the system log....

You can choose universal repeater mode which can compatible with almost all root APs. Either WA901ND or WA801ND has universal repeater mode.

CPE510 as an access point can work with multiple WA7510Ns in client mode. What's the distance from one point to multi-points?

Access control should be able to fulfill. There is an instruction for you, http://www.tp-link.com/en/article/?faqid=359.