Everytime I restart router, i have to plug in ethernet cable again in wan port. Why?

Question

I have an ADSL+2 modem to which I have connected to a TP-LINK WR740N wifi router with a 100 feet Cat6 Ethernet cable. Every time I turn on the router , i have to plug in the Ethernet cable again in the WAN port of router ( Sometimes twice or thrice ), or else it will give limited access and when I open routers configuration page there data packets sent and received are 0. But if I plug ethernet cable direct in laptop it works fine. Only the router issues. I have upgraded firmware of router with the latest version and reset it, but if I put a space character in SSID name, it always gives limited access. Before firmware upgrade router wasn't supporting 100 feet or 80 feet cable at all. This issue happens only on 80 feet and 100 feet ethernet cable. while i tried with 3 and 10 feet cable and it works perfect, never need to restart. What shall I do? All Cables are new, installed recently.