IPv6 router advertisement: advertise only spcific route

Hi there, I have an OpenWrt 19.07.3 box set up as VPN server behind my internet router. The OpenWrt box establishes a site-to-site VPN. Now I would like to confiugre OpenWrt to send IPv6 router advertisement messages to the LAN, but advertise itself as router only for a specific prefix. I did not find any options for this in the docs: DNS and DHCP configuration /etc/config/dhcp

Are there any hidden options for this? Or any packages I can install to enable this?