Cliff, did you change Wi-Fi name of WR740N? Can you detect it and connect to it successfully? Any IP address obtained wirelessly? What's the IP address obtained by wire? Did you try to use it as...

WDS1---Three-Address Frame WDS: It can WDS root router with Four-Addres Frame WDS or Three-Address Frame WDS. WDS2---Four-Addres Frame WDS: It can only WDS root router with Four-Addres Frame WDS. It...

C7 is NOT vulnerable to that attack because of preventing WPS brute mechanism in its firmware, so don't worry...

I got an answer from TP-LINK, “the problem only occurs on ICMP packets which are limited to 2KBytes on SG1006DE; other packets supports 9K jumbo frame ”. Actually, it won't affect your applications...

JurekM, I have the same problem. Contacted TP-LINK support before. The problem is reproduced by TP-LINK technician as well. We have to wait for a solution...:)

No idea about the DHCP issue. Did you try static IP on your PC? WPA281 and WPA4220 can work together indeed.