Sure. As you know a router routes traffic between two or more networks. Typically - at least in our world at Peplink, you will be routing network between devices on a local private network the public internet.
Your internet access router or gateway (be that from your ISP or a Peplink device) uses NAT (network Address Translation) to map requests for internet access from your LAN devices to its (normally) single public IP address on its WAN.
So when you request an internet based resource (be it a webpage or video stream) the remote servers know how to route that back to your gateways public IP. Your gateway then translates the incoming traffic (since it remembers which LAN IP asked for what) to the internal private IP of the device that originally requested the resource. This process means that you can have have thousands of devices all with private IPs on your LAN that connect to the internet using a single public IP (which is good news since there are a limited number of public IPV4 addresses available in the world and we are running out of them). Importantly the firewall on a NAT router needs to be stateful - that is to say it will only allow inbound traffic from an IP on its WAN if a device on its LAN has already initialised a session with it.
When using IP Forwarding on the WAN of a Peplink device NAT is disabled. Instead the router will simply forward on the network traffic to either the target destination (if that device is on the same subnet as the peplink WAN) or the next router based on its routing table. For the receiving router to be able to return the traffic to our gateway it needs to have a route in its table for that network with the IP of a gateway device that can act as a path or route for it. These routes can be manually set with static route entries or learnt dynamically using a routing protocol like RIP.
So lets look at your case.
Assuming NAT is enabled on the WAN1 of the B210, a device on its LAN B will be able to ping a device on the LAN A of the B380.
Send Path: LAN B device (126.96.36.199) -> B210 LAN (188.8.131.52) -> NAT -> B210 WAN1 (184.108.40.206) -> LAN A device (220.127.116.11)
Receive Path: LAN A device (18.104.22.168) -> B210 WAN1 (22.214.171.124) -> NAT -> B210 LAN (126.96.36.199) -> LAN B device (188.8.131.52)
As far as the LAN A device is concerned, the source of the traffic is the B210.
However if a device on the LAN A of the B380 tries to send traffic (or initiate a connection) to a device on the B210 (LAN B) the following happens:
LAN A device (184.108.40.206) -> B380 as its default gateway (220.127.116.11) -> B380 uses static route to forward to B210 (18.104.22.168) -> B210 doesn’t have a live NAT session so drops traffic.
With IP forwarding set on the B210 WAN 1 the following happens:
LAN A device (22.214.171.124) -> B380 as its default gateway (126.96.36.199) -> B380 uses static route to forward to B210 (188.8.131.52) -> B210 Forwards traffic to LAN B device (184.108.40.206).