That is a well-known bug in Windows You can refer to http://www.computertipsfree.com/fix-error-651-wireless-modem-pppoe-broadband-internet-connection-in-windows-7-vista-xp/#ixzz26X1uTxt6 or here is...

you can refer to the following website http://www.tp-link.com/en/article/?faqid=339

then you can also try to reset them by the utility. There is a CD come with the units. Have you install it yet. If not, please install it in your computer. Then reset them by referring to the...

This is a modem that has only bridge mode. So you can try to set up the internet on your computer or the router (if you have one). What is the operating system of the computer? Maybe I can provide...

How did you pair them? You can try to put the two units into a multiple socket and try pair again. You need hold the Pair button 3-8 seconds. Then in 120 seconds, pair the second one by the same way.

it seems that these 2 devices use different POE standard which can NOT commincate with each other. The TL-SF1008P uses Standard POE while the TL-WA5210 uses Passive POE, just check tplink website...