Tue Oct 19, 2021 10:14 pm
The most important thing is to select correct wireless mode for all 3 devices. B should be set as "ap-bridge" (if it's set as "bridge", then it only accepts single client) and A and C should be set as "station-bridge" (any other station-* mode will not allow properly transparent bridging between wired islands).
Not sure what would be proper selection in "quick set" (if there is one at all), so I suggest you to abandon "quick set" mode and start using the "normal" webfig/winbox mode. And never ever go back to quick set mode again. I guess for B anything mentioning PTP is not right (because B is strictly speaking not point-to-point but rather point-to-multipoint or PtMP).
Keep in mind that A and C will not be available as wireless APs (while it is possible to add virtual AP interface, this would greatly reduce the A-B-C bridge throughput which will be hindered already because packets passing between A and C will consume air time twice).