All of a sudden my VoIP phone stopped working. The SPA2102-R and the TP-Link TD-8816 stopped working. In summary, I had to call my ISP. They had to do something on their side. The TP-Link modem then started to work and I had Internet connection. But the SPA2102-R would not work. It had the gree power light flashing. The patter was 3 flashes: the 1st one longer followed by 2 short ones. And I couldn't figure out why on earth it would not connect to the modem. The main indication of the problem was that the LAN light of the modem was off, indicating that the modem and the VoIP adapter were not connected. I called my ISP/VoIP support. They thought that because the LAN light was not turning on that there was some sort of hardware problem with the network card of the VoIP adapter. And they shipped another adapter to me (4 more days without a phone!) The new adapter arrived today. I plugged it in and I got the exact same problem.
I tried different things. I noticed that when I set a fixed IP on the modem and on the VoIP adapter, the light patter on the VoIP adapter changed to almost all lights red flashing. I'm so glad that Cisco doesn't have detailed explanation of what the flashing patterns really mean! So it appeared that the adapter was taking the network settings. I then configured my notebook to be a DHCP server. And I noticed that the light pattern changed (for better): the power green light was solid and the Internet light was flashing (or solid? I can't remember). That was a hint that the problem was not with the adapter, but some detail of the network driver/controller of the modem.
So I left the option that should not have worked but worked at the end: I put the VoIP adapter behind my wireless router. And it worked! It didn't work months ago, because the firewall of the wireless router would block certain (SIP ?) ports. But it worked today (with some changes to the firewall).