Please, i have that problem from buying the router, over a year ago, updates not resolving the problem.

If i set it under 1400, in Speedguide TCP/IP Analizer, it shows correctly, if i set it above 1400, it shows only 1400 every time. I contacted TP-Link, they have no ideea, not solved by updates. What has to be the problem??

From logs, when i change to 1333, it shows correctly in Speedguide:

0days, 00:36:37, [ppp][LCP]RX CONFIGURE_REQUEST
  0days, 00:36:37, [ppp][LCP]TX CONFIGURE_REQUEST
  0days, 00:36:37, [ppp][LCP]TX CONFIGURE_ACK[mru=1400, auth=C023/5, magic=*******]
  0days, 00:36:37, [ppp][LCP]RX CONFIGURE_ACK[mru=1333, magic=********]
  0days, 00:36:37, [ppp][PAP]TX AUTHENTICATION_REQUEST
  0days, 00:36:37, [ppp][PAP]RX AUTHENTICATION_ACK
  0days, 00:36:37, [ppp][IPCP]RX CONFIGURE_REQUEST
  0days, 00:36:37, [ppp][IPCP]TX CONFIGURE_REQUEST
  0days, 00:36:37, [ppp][IPCP]TX CONFIGURE_ACK[ip=10.0.0.1]
  0days, 00:36:37, [ppp][IPV6CP] RX something, but we will not negotiate it.
  0days, 00:36:37, [ppp][LCP]TX PROTOCOL_REJECT
  0days, 00:36:37, [ppp][IPCP]RX CONFIGURE_NAK
  0days, 00:36:37, [ppp][IPCP]TX CONFIGURE_REQUEST
  0days, 00:36:37, [ppp][IPCP]RX CONFIGURE_ACK[ip=*************, dns=*********/*************]
  0days, 00:36:38, [pppoe]ppp0 set localIp ************* remoteIp 10.0.0.1 mtu 1333
  0days, 00:36:38, [wan]advanced ddns -wanChanged
  0days, 00:36:38, [wan]systool sntpc -sntpRequest
  0days, 00:36:38, [dnsproxy]Register Dns Detect
  0days, 00:36:38, [dnsproxy]Register primary = 0x1fce7c1, secondary = 0x17c9ad5
  0days, 00:36:38, [inetd]INETD: select error, abort.
  0days, 00:36:38, [cwmp]cwmp is not start

when i change to 1480,  it shows 1400 in Speedguide:

0days, 00:05:13, [ppp][LCP]RX CONFIGURE_REQUEST
  0days, 00:05:13, [ppp][LCP]TX CONFIGURE_REQUEST
  0days, 00:05:13, [ppp][LCP]TX CONFIGURE_ACK[mru=1400, auth=C023/5, magic=**********]
  0days, 00:05:13, [ppp][LCP]RX CONFIGURE_ACK[mru=1482, magic=************]
  0days, 00:05:13, [ppp][PAP]TX AUTHENTICATION_REQUEST
  0days, 00:05:13, [ppp][PAP]RX AUTHENTICATION_ACK
  0days, 00:05:13, [ppp][IPCP]RX CONFIGURE_REQUEST
  0days, 00:05:13, [ppp][IPCP]TX CONFIGURE_REQUEST
  0days, 00:05:13, [ppp][IPCP]TX CONFIGURE_ACK[ip=10.0.0.1]
  0days, 00:05:13, [ppp][IPV6CP] RX something, but we will not negotiate it.
  0days, 00:05:13, [ppp][LCP]TX PROTOCOL_REJECT
  0days, 00:05:13, [ppp][IPCP]RX CONFIGURE_NAK
  0days, 00:05:13, [ppp][IPCP]TX CONFIGURE_REQUEST
  0days, 00:05:13, [ppp][IPCP]RX CONFIGURE_ACK[ip=************, dns=*************/*************]
  0days, 00:05:14, [pppoe]ppp0 set localIp *************** remoteIp 10.0.0.1 mtu 1400
  0days, 00:05:14, [wan]advanced ddns -wanChanged
  0days, 00:05:14, [wan]systool sntpc -sntpRequest
  0days, 00:05:14, [dnsproxy]Register Dns Detect
  0days, 00:05:14, [dnsproxy]Register primary = 0x1fce7c1, secondary = 0x17c9ad5
  0days, 00:05:14, [inetd]INETD: select error, abort.
  0days, 00:05:14, [cwmp]cwmp is not start

I put some stars there... Whan can be the problem and how to resolve?