@tpta Before you start I would suggest to backup the TD-W9960 and C80 configuration just in case. You will need these credentials for configuring C80 PPPoE session: Delete the old Internet connection...

@tpta OK, but you should know that NAT loopback/hairpinning is a feature that is FW embedded. If it's not implemented in the C80 current FW release (which can be confirmed by TP-Link), then you'll...

@tpta From your description it seems like NAT loopback/hairpinning on C80 doesn't work. Why don't you try to put C80 in AP mode. Thus you'll avoid the double NAT in your network and probably the...

@Archernew Get the latest FW from here. Update the C54 FW and test again with C54 DHCP server disabled.

@Archernew OK. The last screenshot shows that you have 3 IP addresses assigned: 1-st from main router DHCP server 192.168.1.4 2-nd 192.168.1.101 - that's the C54 IP address 3-rd from C54 DHCP server...

@Archernew In RE mode the C54 DHCP settings should be grayed out (DHCP server not available): Make sure you are with the latest FW for C54. If not, then upgrade to the latest FW release.Keep in mind...