Understood. Seems like access point mode is an area in open source based wireless "routers" that is not as straight forward as one would often hope.
I know, they are called "routers", which is how they are likely used by the majority of consumers. However, not everyone needs a layered network (hierarchy of routed NATed subnets) in their house, having everything on one level is much simpler for some, especially in the are of IPv6 propagation downstream for the public facing firewall/router.
I guess one could achieve a "flat" (same subnet network) by the use of OpenVPN tap mode (level 2) VPNs, but that is a lot of work to achieve the same functionality of dumb AP mode.