What is the WAN IP address of the secondary router?
What is the subnet of the secondary router?
If you do not know or are unsure then Please connect to your secondary router using ssh and copy the output of the following commands and post it here using the "Preformatted text </> " button:
Remember to redact keys, passwords, MAC addresses and any public IP addresses you may have:
ubus call system board
cat /etc/config/network
ifstatus wan | grep -i address
WAN IP is 192.168.5.146
Subnet of secondary router is 192.168.6.0/24
Secondary route IP is 192.168.6.1
On the main/primary router you create a static route routing 192.168.6.0/24 via 192.168.5.146:
Make sure to reboot both routers
Note not all LAN clients on the secondary router will allow traffic from a different subnet (192.168.5.0/24) as they can have their own firewall which needs to be tweaked.
Alternative method is to enable Masquerading on the LAN zone of your secondary router, you can do this to test if the LAN clients firewall is the problem
Well it is currently working, but I'm not really sure why. I'm just leaving it alone for now as I either had access to the internal devices or access to the internet, but not both. Right now I have both so I'm not touching it again until the weekend and hope nothing causes the router to reboot 
Great to hear it is working.
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Thanks! 
Yes I will. Not convinced it is totally solved, but not touching it until the weekend. I'll post exactly what I did.
I'd done just a static route before and it hadn't worked, so I'm not sure what I have done differently this time.
