Updating to the latest firmware version resolved the problem. I would like to re-emphasise the fact that my router was hardware version 2 not 1 to anyone experiencing similar problems.

Ada wrote You can refer to the table in instruction below. http://www.tp-link.com/en/article/?faqid=340 mp4 is the supported format. BUT C7 doesn't use standard DLNA, which may cause some compability...

Region : UnitedKingdom Model : Archer C7 Hardware Version : V1 Firmware Version : Initial Realease Version ISP : Virgin Very simple problem, I have connected up a USB hard drive to my router for use...