I have the same issue with my C5, you can try disabling the "NAT Boost" option. After this loopback works for me. However, the router's performance also drops in this case (max 300 mbit/s)

Finally I took it back to the shop where I purchased it, and they changed it to a new one under warranty. Looks like it was a hardware problem.

Model : Archer C7 Hardware Version : Not Clear Firmware Version : 3.15.1 Build 160616 Rel.44182n ISP : Telekom Hi All! I have an Archer C7 (HW version 2.0) that has been working in the past 1,5 years...