gladly. please email support and try to arrange time in GMT working hours, if possible.
Guys,
After some more digging I was finally able to discover how do I get in my LAN those weird IPv6 prefixes from.
It turned out that my Nest Thermostat is rarely sending IPv6 RAs, offering some IPv6 ULA prefixes, causing my devices to use Nest Thermostat’s ipv6 link-local address as their default IPv6 gateway where Nest is definitely not a router. I found that all Nest devices like Smoke detectors etc are doing the same thing.
This combination makes my iPhone XS basically not operational when connected via MikroTik AP set to 802.11ac. It is working fine when MikroTik AP is set to 802.11an. I believe due to some unknown bug in processing 802.11ac packets, MikroTik is preventing the iPhone XS (and most likely the new 2018 iPad Pros as somebody reported) from fallback to IPv4. Maybe it is dropping IPv6 packets, that I don’t have time to check.
All my other devices like iPad Pro 10.5” (2017), iPhone X, MacBook Pro (2017), MacBook 12 (2017), iMac 5K (2015), PC with Windows 10 don’t have problems with connectivity even with those silly IPv6 addresses and gateway assigned. Only the iPhone XS and only on the MikroTik’s AP configured to 802.11ac. I have other Asus 802.11ac AP in the same bridged network and when I’m in its range my iPhone XS works perfectly fine, even tough I still have those IPv6 addresses assigned from Nest Thermostat.
As a temporary workaround I have connected my Nest Thermostat to an isolated Guest SSID on my Asus router, so it is no longer advertising those IPv6 addresses to my home network.
Immediately I got rid of this crazy problem and can enjoy my 866Mbps connection via MikroTik’s AP!!! 
To MikroTik staff : If you want to reproduce this issue I suggest you connect some Nest device to your network as the only device that advertise IPv6 prefixes and connect your iPhone XS to it using 802.11ac.
You have to wait couple minutes because Nest sends RAs every few minutes or so.
If you have other routers sending IPv6 advertisements you may still not be able to reproduce this issue, so make sure Nest is the only source of IPv6 in your LAN. Your main internet connectivity still has to be over IPv4. Make sure you’re using FQDNs in Safari when testing connection. If you don’t have any Nest equipmnent you might try to connect additional router to your LAN which will only send IPv6 RA’s and ULA prefixes but will have forwarding disabled - sort of dead end IPv6 router.
Flowers and bottles of bourbon are very welcome, if you wanna send one PM me for an address 
Best regards,
Greg
Btw. This problem still existed on latest iOS 12.1.1 and on latest RouterOS 6.44beta40
Greg,
Good job tracking this down; however, I do have a question. We have nine (9) Nest thermostats. In addition, we have 11 Nest protects. We have a static IPv6 /64 address space that is fully routable. Every device that is configurable has a manually assigned IPv6 address. This includes all MT, Macs, Sony TVs, etc. As you are aware, we cannot configure IPv6 on the IOS devices. Our Core MT is the default route and most every device shows that router as the gateway, and in the case of “self assigned” the address is using the proper prefix. We pass a lot of IPv6 traffic. Since my XS devices are showing a correct public IPv6 address and gateway, it must be the LL address that are messed up. Did i get that right?
I am going to reassign all Nest devices to a new VAP and see if that solves the problem as you suggest.
Comments?
Thanks,
-Scott
Scott,
If you have a proper IPv6 default gateway (your edge router) on your iPhone XS then it would mean that the problem is actually forwarding IPv6 packets on MikroTik’s 802.11ac and has nothing to do with Nest in your office. In my case Nest is the only source of IPv6 in my LAN. Once I got rid of the IPv6 my iPhone XS and MikroTik’s 802.11ac works beautifully. Usually when there is a problem with IPv6 the CPE should fallback to IPv4 but somehow it’s not happening and everything is stuck.
Can you text me on Skype - freezer_szczecin
FYI, I’m based in Poland and got +9hrs to pacific time.
Greg
Greg,
I am presently in Whistler Canada on a short holiday. I will
be back next week and will reach out to you.
I am generally located in the San Francisco CA area.
-Scott
If you have single AP, can the problem be solved by disabling client-to-client forwarding on wifi?
Chupaka,
Do you also have Nest in your LAN or any other source of IPv6 ?
Greg
Nope, neither Nest nor iPhone
Chupaka,
So what CPE/UE do you have ? This discussion is all about connectivity issues on new Apple CPEs like iPhone XS or 2018 iPad Pro.
Hmmm IPV6 package not enabled on our clients router, also no Nest devices. However setting 5Ghz to wireless N does work. Though not ideal.
I understand that, I’m just trying to guess whether client isolation on AP can be a workaround for the problem.
I plan to make some changes to the firewall to drop all ipv6 communication from the Nest Thermostat it seems a reasonable workaround for this issue
Ofer,
If you have only one AP and the firewall is processing bridged packets then blocking ICMPv6 should do the job.
Chupaka,
If client isolation blocks all the multicast traffic between clients then this should also do the job.
I thought about dropping all IPv6 packets from the Nest across all my routers, I don’t use IPv6 anyway and I removed that package from all the routers.
Ofer,
It doesn’t matter if you have IPv6 package or not because it is pure layer 2 transmission (like broadcast). Actually having IPv6 package might help with setting up some IPv6 firewall rules
Just stating the details but I’ll try to configure it to drop IPv6 packets I’m curious to see if it’ll resolve it.
Hm interesting. All my IoT devices use another SSID, the main SSID ir only for phones and computers. If you all do the same, do you still see the issue? Do all of you actually have NEST devices?
I do have the Nest Thermostat so I can check and see if it’s actually the cause of the issue.
Seems to be reported long ago - https://forum.archive.openwrt.org/viewtopic.php?id=54161
Normis,
Bare in mind that Nest in this case is just an external factor which arises some bug in MikroTik software, because same setup with my iPhone XS with same IPv6 ULA prefixes coming from Nest is working fine on ASUS router.