Issues with IPv6 disappearing after a while

I have some issues with a certain ISP, they provide IPv6 through DHCPv6 and while I have configured the correct VLANs for IPv4 and IPv6 internet, and both get delegated addresses, the IPv6 disappears at some point and even restarting the interface doesn't bring it back. Only a reboot of the router does in most cases, but not all. I tried looking in the system log for reasons why this happens, but can not understand why the router gets IPv6 but then loses it. Each reboot comes with a new IPv6 block. Any suggestions what I can look for or what I can try to remediate this issue?

Could be some lost responses from the dhcp or RA.
The output of logread when it happens may give some insight for a start.

Thank you, I will look into it, since I am not sure when it disappears should I just save the log to some place and then try to figure it out? What should I be filtering the log for?

You can paste it here, or if it is too long in pastebin. Better not filter anything for now.

What's the best way to save the log? Just change the log output to the SD card?

logread > /tmp/log.txt
Then copy it from the router to your pc by scp.