Turning off NAT Boost in my router settings has enabled loopback to work and solved my issue.

@Lumin1a On my firmware version, I don't have a "NAT Boost" option at all.

Thank you for the suggestion though.

@An0n I know this thread is already over 6 months old, but I've had the same issue with my C5400X and am trying to figure out if there is any way to enable it. Did you have any luck figuring it out? I would have thought a router this expensive would have higher level networking functionality like this but maybe I'm just looking in the wrong place.

@Quartermain Not that I know of unfortunately :/

@Quartermain You're in the right place, unfortunately. We love TP-Link routers for their stability, however the firmware is lacking.

I was able to get NAT loopback partially working with my C5400X. The caveat: I had to access the device (LAN) via wifi. Kind of ridiculous, but it works until the firmware is fixed. :)