DHCPv6 Settings for IPv6 Setup on Synology WRX560 Router

Good afternoon:

I am using the Hitron CODA56 modem with a Synology WRX560 router with two networks: Network 1 (Primary Network) and Network 2 (Guest Network). I need assistance on enabling IPv6 for both of these networks.

In Network Center > Internet > Connection > IPv6 Setup, I've set the "IPv6 Setup" drop-down to "DHCPv6-PD;" and in Network Center > Local Network > Network 1 (Primary Network) > Edit > IPv6 DHCP, I've checked the "Enable IPv6" checkbox, selected the only available prefix from the "Prefix" drop-down, and selected "Stateless Mode." In doing so, the devices on Network 1 (Primary Network) now pass https://test-ipv6.com with a score of 10/10.

I attempted to do the same for my guest network by navigating to Network Center > Local Network > Network 2 (Guest Network) > Edit > IPv6 DHCP. After checking the "Enable IPv6" checkbox, I attempted to select a prefix from the "Prefix" drop-down, which gave me an error: "IPv6 prefixes are used up. Please go to Internet > Connection to check the IPv6 prefix settings. Alternatively, you can release prefixes from other local networks at Network > select a network > Edit > IPv6 DHCP"

So at this time, I only have one prefix available to select, meaning I can have IPv6 on either my Primary Network or my Guest Network but I cannot have them on both. Is there a specific setting/configuration I should be using so I can be given enough prefixes to let these two networks have IPv6?