OPNSense behind ISP Router, can't access devices on ISP lan without double NAT

[Niami]

I'm trying to implement an OPNSense router into my homelab, A for testing and B to eventually replace my ISP router.
B is easier said than done so for now I have my OPNSense behind my ISP router

ISP Router A - 192.168.1.0/24
         |-------------------------Other devices
OPNSense Router B - 10.20.10.0/24
         |
       PC

Router B is set up so outbound NAT is completely disabled and I have a static route set from router A to router B to forward all 10.20.10.0/24 traffic to router B WAN. For testing purposes all interfaces have an any/any pass rule just to rule this out for now.

However with outbound NAT disabled I can access the internet but not devices on the Router A LAN from the Router B LAN. I can ping the devices, but cannot access any dashboards through http. Enabling full auto outbound NAT solves this. What have I missed or not understood?