So on DD-WRT, Assign WAN port to switch makes a router a 5 port switch. It still has an IP address but the devices behind it just get handed off to the main one.
I tried to do something similar in LEDE yesterday but failed hard. I ended up locking myself out of the router by setting br-lan as a DHCP client.
Anyways, is there an easy way to do the same thing in LEDE?