In router mode, it still assumes it will be placed between two different subnets and need to do routing between those two. So its by design that it works on level 3. A gateway is even worse, as it does port mapping and firewall.

What you really want is a simple non-routing access point with built in 5-port switch. If you dive deep into the DD-WRT config, you can make it like that, I am sure, but its not one of the pre-set choices. But you’d have to dive deep, so best not to bother and be happy with your 4-port switch