I’ve unboxed a brand-new hAP mini, booted it up connected to the Fritz!box that serves this location with internet access and WLAN, upgraded to the latest stable RouterOS, rebooted, and then used the 「Setup Repeater」 button to set it up as WLAN repeater. (And disabled the DHCP server…)

Now I can switch my laptop between the Fritz!box and the hAP mini by using:
(MAC addresses and SSID redacted, of course)

However, as soon as I do that, my IPv6 connectivity is gone. Worse, if I I only get one duplicated response back, indicating that the hAP mini is not bridging as it’s supposed to.

How can I fix that? IPv4-only is absolutely insufficient.

True wireless bridging is not standard feature of 802.11. Different vendors came up with different workarounds which work well only when main AP and "repeater" talk same dialect (mostly that means they are both made by same vendor).

I don't know how exactly QuickSet configures "repeater mode", but I'm guessing "station-pseudobridge" which works somehow for IPv4 but doesn't work for IPv6. You can read pretty good explanation of the problem in this article.

Thank you.

This is sad. Indeed, switching to station bridge loses connectivity, so AVM and Mikrotik don’t speak a common dialect.

Well, the Fritz!box is… not very good anyway, so maybe we’re going to replace the WLAN part of it, at least, with another Mikrotik box. (Probably would like to do more, but we need its VDSL, VoIP→POTS and DECT parts still…) Then we can bridge between these.

When you buy MikroTik again, please do not choose a hAP mini.
It is a nice toy but not useful for real usage. You can keep it for doing special things like having a Wireless client on a device that has only Ethernet.
Whatever you do, do not upgrade it to version 7.xx

When looking for mikrotik wireless devices to build wireless bridge beware of this: there are two wireless drivers, wifiwave2 which only works with newer wireless chips (some ac and ax) and legacy which works with all except ax chips. wifiwave2 brings much better performance on supported chips but currently doesn't implement 4 address mode which is needed for wds and *-bridge modes (MT said that 4 address mode is on ToDo list but without any ETA). Legacy wireless driver suports 4 address mode (and a few other functionalities), but at sub-optimal speeds. And can't drive newest ax devices. Which means you either have to wait for 4 address mode implementation in wifiwave2 (could be months or even years) or decide on a slightly older devices with ac chips and build wireless bridge today. If you decide for the later option, you may be able to change to wifiwave2 driver later, but device has to fulfill the following specifications: all radios are built around ac chips (RB4011 fails this criterion), have at least 256MB RAM (hAP ac2 fails this criterion) and have at least 32MB of flash storage (my own experience says 64MB is minimum; hAP ac2 fails also this criterion).

Given the sad state of MikroTik indoor wireless, and the fact that the OP already has a Fritz!box, he is probably better off buying the AVM WiFi extender.

True wireless bridging is not standard feature of 802.11. Different vendors came up with different workarounds which work well only when main AP and "repeater" talk same dialect (mostly that means they are both made by same vendor).

Ran into the same problem today - no IPv6 over station mode pseudobridge.
IEEE 802.11c from 1998 specifies wireless bridging, it was integrated in 2004 into IEEE 802.1D - why would Mikrotik not support this?

True wireless bridging is not standard feature of 802.11. Different vendors came up with different workarounds which work well only when main AP and "repeater" talk same dialect (mostly that means they are both made by same vendor).

Ran into the same problem today - no IPv6 over station mode pseudobridge.
IEEE 802.11c from 1998 specifies wireless bridging, it was integrated in 2004 into IEEE 802.1D - why would Mikrotik not support this?

And why wouldn't they implement the few Wireless Bridge vendor implementations? My Asus n router can wireless bridge to my Linksys ac primary router. Two different vendors, both using a supposed "non-standard" mode, but functioning fine in this mode. There clearly aren't that many different implementations.

Mikrotik chooses to only implement their proprietary version when I thought the entire purpose of their routers was flexibility. I bought a hAP AX2 for the purpose of wireless bridging or repeating and everyone said this router could do everything because of the OS customizability. In reality I would be better off just buying from TP-Link or Linksys. It's just like Linux where everyone says how powerful and great it is but when you go to actually use it, there's a deal breaker like broken audio (because the Linux community couldn't possibly support as many devices as Microsoft does) that makes it completely unusable. This isn't even mentioning the fact that half the time you make a config change you get locked out of the router config and are forced to factory reset.

And yes you need IPv6 and have needed it for years as many high profile websites slow down significantly when limited to IPv4.