DMZ makes no difference.

In reply to a post by Raedwald512:

Having some "interesting" issues trying to connect a laptop to the office via VPN and would appreciate any assistance.

If I connect via the home network, either wired or wifi, then without fail I get an error - host unreachable. Internet access is fine, it just wont connect to the VPN.

If, however, I connect via wifi to the BT WiFi with Fon network on the same HH5 then, once I've signed in to BT via a web browser, the VPN connects almost immediately.

Other than the network name and internal network IP range, no other settings have been changed on the HH5.

Hub version: BT Home Hub 5 (Type A) | Software version 4.7.5.1.83.8.204 | Last updated 22/01/15

Ideally, I'd want to connect via the home network (security) rather than the BT WiFi one (unsecured), but I can find no way to make it work.

Five paragraphs in your mailing so let's pick them off one by one.
1) OK let's see if my two penny worth helps.
2) Right then if you use the Command Prompt with ipconfig, what are the values for IPv4 address, Subnet Mask and Default Gateway? Do you get a response if you ping the remote site? Also use www.ipchicken.com to discovery what your WAN address is.
3) When connecting via BT Wifi your IPv4 address, Subnet Mask and Default Gateway will be different from 2) above. But is the WAN address different? Since you can establish VPN connection this suggests that the remote site might only accept connections from a know range of WAN addresses. Was there a requirement to supply your WAN address? If so you might need a fixed IP.
4) It might be the case that some ports on your HH5 might need to be opened when using your LAN. Seek advice elsewhere on that one please.
5) No comment.
6) But of course and it may well be the HH5 is stopping two way traffic by blocking incoming responses from the remote site onto your LAN. Since you seem to be absolutely sure that the credentials for connection are correct, would it not be possible to visit a neighbour/friend/whatever who does not use BT Internet and see if you can establish a VPN connection from their LAN? If it works there, then that's one box ticked as OK to move onto other diagnostics to determine actions to be taken.

It could be at the end of the day you may have to ditch the HH5 for another router but that means sourcing an Openreach modem if that is not part of your installation.

In the office I use BT Internet but have a Draytek 230vN via the Openreach modem and have no problem establishing a VPN connection from home using my LAN.