The router's speed you see is synchronous speed , which should be a bit higher than what ISP provides you. You should confirm if your ISP provide a more than 10 Mbps bandwidth.

Technically, when you disable WMM, the router automatically change to 11g, that's because the function itself can't run on 11n. Some TP-LINK Routers allow disabling WMM in 11n mode, however, it will...

You can use your friend's modem to have a try, if normal, just ask TP-LINK for an exchange, but if the same issue, nothing to do but contact your ISP again, just as they said, you are too far from...

If your want to use two APs to WDS 8970, you should configure on both of the two devices to connect to 8970, not configure on 8970 to connect others.

Attention! You can only upload firmware of V4, and impossible to upload V5

You can only upload a firmware with a cable(by wired) ,rather than Wifi.Maybe that's the problem.