@NeonBones 

Hi,

The Archer C2600 has two USB ports, while the A2600 shown on TP-Link's website has none.
Therefore, the two routers might actually be based on entirely different hardware.

Going by appearance and specifications, the "Archer A10 V1" seems to be closest to the Archer A2600.

https://www.tp-link.com/us/support/download/archer-a10/v1/

But I wouldn't try to flash a firmware made for the Archer A10 onto the Archer A2600, as this could render the router unusable.

  @NeonBones 

Hi, there is indeed an Archer A2600 available on JP site: https://www.tp-link.com/jp/home-networking/wifi-router/archer-a2600/

  @woozle thank you for the reply, good to know. I was looking to get a VPN running through the router but it looks like that isn't possible without a firmware update. Don't suppose you have any advice on that?

All good if no, just thought I'd ask!

  @NeonBones 

You should only update with a firmware file that matches the router's model description (as printed on the label on the underside of the router).

According to its specification the Japanese retail A2600 should already support OpenVPN server and PPTP server out of the box. https://www.tp-link.com/jp/home-networking/wifi-router/archer-a2600/#specifications 

Is there any chance your A2600 might not be a retail version, but instead was supplied directly by an Internet Service Provider?