I have a Vigor 2865ax behind a Technicolor DGA4134 Router - on which DMZ is active on Port 4.
Technicolor Router has a fixed external IP address, the internal IP Address is 92.168.50.254 and it's DHCP allocated internal address scheme is 192.168.50.xxx/24.
Some devices are connected by wireless to the Technicolor Router - those devices which have no need to access the "Work" network on the Draytek router.
The Vigor 2865ax is attached to the Technicolor Router via Ethernet Port 4 and has an IP Address of 192.168.50.15. 
The Vigor 2865ax's Internal IP Address is 192.168.10 254 and it's DHCP Server is shut off.  All IP Addresses on the internal network are furnished by my Windows Server 2022.
Everything works perfectly on the network, whether connected to either the Draytek or the Technicolor Router.  
I cannot get the Draytek VPN Client to recognise that the Vigor 2865ax router exists when attempting to attach via Draytek VPN Client from anywhere outside the house.
It doesn't matter what protocol I try - simply doesn't work.
Oh, and additionally, when I try to use the Draytek router directly on the VDSL (by cutting out the Technicolor router) it will not connect to the VDSL. It appears not to like the GFast requirements of the VDSL
Local ISP is Manx Telecom - easily not the best there is, but unfortunately they have a monopoly on this island and it's all we have to work with.
Anyone else have these issues?  Can anyone help me try to get my VPN Client working, please?