Try ebay. Or buy a pair and sell the spare on eBay.

It is a V1 or V2 7020? What firmware version?

Most cable / ADSL modems also route and will issue DHCP addresses by default. It's been a long time since I have seen one that doesn't. But yes if its a pure modem device that just passes on 1 IP...

Try and connect to http://tplinkplc.net/ via your browser If that give a login to the TP-Link WiFi device then you are connected to that.

" (3) I don't think the 8630 has all the capabilities for a router - it would have to have dedicated WAN mode for one of its RJ45 ports, it needs to act as DHCP server, and many other things it...

Make sure you are a) running the latest firmware and b) plugging directly in wall sockets and not using extension leads. Also try different sockets if that's an option.