I don’t know if that would fix the problem, but:
- don’t create IPv6 pool manually. DHCPv6 client will create it automatically after it receives prefix.
- don’t use prefix-length=48 (either set it to 64 or omit it altogether), it doesn’t do what you probabky think it does. It’s about prefix length when they are handed out: either to DHCPv6 server if your router is set up to pass prefixes further) or when assigning ipv6 address using syntax address=::1 from-pool=.
If you want to indicate desired prefix size to upstream DHCPv6 server, you can set prefix-hint=::/48 property of DHCPv6 client (but it’s a hint and upstream DHCPv6 server is likely to ignore it).
It could be that your ISP is handing out smaller (longer) prefixes than /48 (mine is giving /56, some are giving /60 and some brain-dead ISPs are giving /64) and since your setting says you’ll be using /48 prefixes, any received prefix smaller than /48 might be ignored by DHCPv6 client.
Also: setting add-default-route=yes is conceptually wrong, but it might work with some ISPs. What this option does is that it takes DHCPv6 server’s (link-local) address and uses it as upstream gateway in default IPv6 route. This works fine if DHVPv6 server is a router as well (either actually intended one or just happpens to support routing), but thr correct way would be to tisable this property and set /ipv6/settings/set accept-router-advertisements: yes (default is effectively “no” while ideally that would be enabled per interface).
But the setting, discussed in previous paragraph, doesn’t prevent your DHCPv6 client from obtaining prefix.