WPA281KIT and WPA4226KIT can work together and the rate between them will be limited by the one with slower speed.

Reset both to factory defaults using the utility. Connect them in the same room and check the rate displayed in utility.

All devices connecter to the extenders lose connectivity frequently, through wireless or through wired connection. How about status of all lights on powerline adapters? How did you make them work...

What's the schedule you set? Did you set system time first on Management---Internet Time page?

my version is 5.1, I downloaded version 6 ras file Not right. You should download the latest firmware (TD-W8951ND_V5_141114) for V5. Remember to extract it before you upgrade it....

Josef, did you try different web browsers? Did you read the notes for TD-W8961NB_V3_140605? http://www.tp-link.com/en/support/download/?model=TD-W8961NB&version=V3#tbl_j Can you upgrade the firmware...