There is some misconfiguration here, because the lan interface doesn't get any prefix. Then you can assign ::1 to that and it will be preferred source for curl and others.
Please run the following commands (copy-paste the whole block) and paste the output here, using the "Preformatted text </> " button:
Remember to redact passwords, MAC addresses and any public IP addresses you may have
ubus call system board; \
uci export network; \
uci export dhcp; uci export firewall; \
ip -6 addr ; ip -6 ro li tab all ; ip -6 ru; ifstatus wan_6