How to announce offlink prefixes?

Installing and Using OpenWrt Network and Wireless Configuration
1

I have 2 routers in a broadcast domain: 1 acts as default gateway, while the other acts as VPN router to another site.

I want the VPN router to advertise itself as router for prefix 2001:db8::/48. How do I do that in /etc/config/dhcp?

2

I think odhcp6d and dnsmasq can not announce additional routes. But bird2 or radv can be configured to do so.