Wi-Fi does not care (too much) about IPv6, as it is just Ethernet on that layer. The only problem for any carrier of IPv6, be that normal wired Ethernet or not-wired Ethernet (aka Wi-Fi) is primarily Multicast. This due to Neighbour Discovery. Nothing special is needed for any Wi-Fi device though. But like switches that do L2 MLD snooping, that kind of construct can also conflict with IPv6. The device that handles the actual routing needs to support IPv6 explicitly though. That said, Wifi "repeaters" and "extenders" are typically a really bad idea: they at minimum cut the throughput in half, as it receives and then re-sends in the same frequency band. A Raspberry Pi, or any other such solution, would have the same problems, just deploy one with two WiFi-sticks, let one play AP and the other a client to the network you are trying to extend. As per above: you lose a lot of bandwidth that way.... Better to just pull an Ethernet cable to a second Access Point and let it properly distribute the Wi-Fi signal there. Or if you can't pull a wire, get a proper access point, for budget minded, the Ubiquiti AP AC Pro comes to mind, which is a very normal Layer 2 Wi-Fi Access Point and thus does no routing of any kind, that is for a router to do....