Sonicwall / Router / VPN

Hi does anyone have any advice to get the following working.

We have a draytek vigor2600 router with passthrough active this goes to a sonicwall tz170 which goes to PC.
We want the pc to run VPN. Someone needs to connect to our pc remotlely with VPN.
However we have a single static IP address which the draytek takes , it assigns a IP to the sonic wall.
The person connecting uses the static IP address , they can see the sonic wall and chnage the settings etc but when it comes to the vpn it wont work as the sonic wall is not using the static ip.

Any Ideas We are using BT Business with single static IP.
We do not want to have to pay for more static IPS.
And we cant just use the draytek even though it has a firewall because the company connecting require the use of the sonic wall.