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Prefix delegation won't work if higher router send IA_NA, only IA_PD seems to be supported.
Not sure you can influence this on your Fritzbox, but it helps to set:
Minimum RA interval = 200
Maximum RA interval = 600
Lifetime = 7200
The router needs to advertise a prefix larger than /64, e.g. /62, /60, /56 etc.

Your parent interface needs to have NO ipv6 address (no autoconfig, not anything else).
Your child interface needs to have "obtain via prefix delegation).
Policy needs to permit protocols required on child and upstream interfaces / zones.

dhclient would write log to /var/log/messages which you can access via syslog / expert mode.

See some comments I shared previously on this subject.