Very simple answer is relayd.
Routers with a single radio don;t seem to like to do what you want.
Start here, but there are a number of articles to read: Routed Client with relayd (Pseudobridge) [Old OpenWrt Wiki]
Different chip sets support different capabilities, and I do not know what this supports, but for my RT5350 this was the route.
Or you can install a USB radio and simplify things.