How to configure SLA ID with IPV6 PD (Firewalla Gold SE)

I would really like to know how to configure an SLA ID per network with IPV6 PD. Right now I have 2 networks which on reboot of the router randomly get's either the prefix (SLA ID) 0 or 1 in addition to the /48 range which I'm getting from my ISP through PD. I would like this to be static per network while still using the IPV6 PD. Does anyone now how I can configure this (alternative is to set IPV6 to static on both networks which I don't really like)