"I don't understand what you mean by "instance in the Network -> VLAN -> Port Based section of type 802.1Q for eth1 interface""
Sorry, my bad. I meant to say interface-based, not port based.
Anyways, Based on the network topology, it looks like you don't actually need to use a tagged VLAN on the WAN. Instead, you can simply configure the WAN port as a LAN port.
Then, you can set up port 3 to be a port-based VLAN (LAN_P) on the 192.168.2.0/24 network.
This means that LAN ports 1, 2, and the WAN will all be on the 192.168.1.0/24 network, while port 3 will be on the 192.168.2.0/24 network.