Is your second router configured as a bridge or a separate subnet?
If it is a separate subnet I don’t think there is any way to properly support IPv6 for the devices connected to the second/third router.

Reason being that AFAIK, you only get a /64 IPv6 subnet Prefix from the ISP for your 1st router, which is not supposed to be subnetted any further.

I don’t know if any of the ISPs will allocate something like a /56 IPv6 prefix to home fiber users. Actually I’m interested in the answer to this question myself. I planned to move to a new place soon, so I didn’t bother to check with my current home fiber provider.

See https://serverfault.com/questions/714890/ip...-work-around-it

i don't know enough about ipv6 but from my high level understanding u should try like this

connection type : passthrough
dhcp-pd : disable
connect to dns server automatically : enable
router advertisement : disable