The routed client requires the wan as a separate subnet, since a regular AP-STA connection doesn't support bridging the STA end. Basically your router acts as one device on the WAN network and uses NAT to have multiple LAN users pass through that one device.
But that isn't why it can't run AP and STA at the same time. That is a limitation in the driver as it's a Broadcom chip. They have poor open-source support.
The Pi 4 is cheap and has a lot of memory and CPU power, but it isn't made for hardcore networking like something purpose built as a router.