OK so basically you want to change the WAN port to be a LAN port. Yes this is easy (if your router supports it), you only need to go to Network > Switch
The image below shows example of my default config on my router.
And this is after changing the WAN port to be part of the LAN VLAN
To achieve that, you will need to identify the WAN VLAN (usually VLAN 2, but generally it's the VLAN that has the WAN port untagged and all LAN ports off), delete it, or just change the WAN port from untagged to off, then for the other VLAN (the LAN VLAN), change the WAN port form off to untagged.
Numbering will vary between routers, but simply you want one VLAN to have all 4 LAN ports + the WAN port set to untagged.