AX5400 IPv6 randomly stops working on connected device

[LordSnape]

Hello everyone.
I am hoping for some help, because this issue has been driving me nuts for a while now.
My AX5400 is running on 3.0.0.4.388_22525-gd35b8fe, was on 386-something before. The bug persisted, the only reason I upgraded was in hopes it would magically fix itself. Also, I have recently done a fresh totally clean reinstall of Windows on the PC, the issue carried through it.
Anyways, the actual issue is the following:
My IPv6 configuration is working correctly (afaik), because most of the time it does actually work. My PC is connected through wire and successfully gets assigned an address from the /64 pool my router has.
But sometimes, and I have no idea what the pattern is, it just stops working on my PC. The address is still there, it doesn't get unassigned, the lease doesn't run out (it can work fine for days or break after 15 minutes) etc.
I have tried pinging "LAN IPv6 Link-Local Address" from my PC and it is still accessible. However, "LAN IPv6 Address" is not. When everything works fine, both can be pinged.
The "WAN IPv6 Address" router assigns itself randomly from the /64 block is still ping-able from the outside world when my issue occurs (which means there still is access to the WAN).
Usually, rebooting the router fixes the issue. Sometimes I have to do it multiple times.
What could be the cause of this and what's the best course of action? Thanks in advance!

 

[L&LD]

Welcome to the forums @LordSnape.

If you didn't do a full reset to factory defaults, followed by a minimal and manual configuration of the router to secure it and connect to your ISP, and without using any saved backup config file, you probably need to do that now to get your router to a good/known state again.

[Wireless Router] When Standard Reset Isn’t Working: Hard Factory Reset - Models list | Official Support | ASUS Global

Follow the link above to properly reset your router and be sure you're following the instructions for your particular model.

 

[LordSnape]

Follow the link above to properly reset your router and be sure you're following the instructions for your particular model.

Okay, I have just finished doing that and reconfigured everything from scratch. Highly doubt it will help, but so far it works (but it did before as well).

Edit: Nevermind, it happened again. Had to reboot the router to get my IPv6 working once more.

 

[L&LD]

Okay, what are your steps to reconfigure everything from scratch then?

 

[LordSnape]

Okay, what are your steps to reconfigure everything from scratch then?

I did it through the estup panel that shows up on the first boot?
I entered my PPoE credentials from the ISP, set the Wireless password and enabled Native PPP IPv6 in the IPv6 settings. After that, reconnecting to the network from my PC. IPv6 got linked up and it was working, until it randomly stopped again. Then I rebooted the router. IPv4 is working fine all the time.

 

[L&LD]

Then you need to get a hold of your ISP and see what may have changed (on their end).

Or, leave everything (including any modem/ONT or their end) powered off for an extended period and see if that fixes things. By 'extended', I would be looking at 10 minutes minimum, but up to just over an hour.

 

[LordSnape]

Could it actually be my ISP's problem, if the "WAN IPv6 Address" my router assigns itself is still accessible, without the reboot?

 

[LordSnape]

Don't want to leave this thread hanging, so will sum up what I did so far.
My ISP gives me two almost identical /64 IPv6 blocks, one being subnet a, other being subnet b. Apparently while both "work", subnet "a" was meant for the router as WAN subnet, and the other subnet "b" was meant as a LAN subnet (even though it's not actually LAN - those addressed go to the internet, after all?). It has been working fine for a week after I changed settings to reflect that.
Hope this can help somebody in the future.