We have two subnets in our office (one is used for setting up kit on the range on the intended range of wherever its going)

We have a 2820 managing that subnet with DHCP in one range and a main 2820 doing our primary subnet again with its own DHCP.

the second 2820 has no internet connection but gets its internet of the first by running a patch from the main lan to its WAN2 port and having its IP set dynamically.

This allows everything on LAN2 to see everything on LAN1 but not vice versa as its not routing traffic from LAN1 to LAN2. Is their anything I can do to alter this behavior? Ideally I want both subnets to be able to communicate though (pure routing) but assumingly its setting it up as NAT.

Have you set up a static route on the LAN1 router? That would make it aware that your LAN2 network can be accessed using the LAN2's IP address on LAN1's network.

Yeah their is a static route but the other draytek is just refusing to route traffic though. Ive even put the subnet in as a list of "public" ips but still no joy.