If you use the Zyxel as a bridge, don’t you have to look for the solution on the UDM Pro?
However, I don’t know whether the following support document helps or whether it works because I don’t have my own UDM:
[https://help.ui.com/hc/en-us/articles/115005868927-UniFi-UDM-USG-Configuring-DHCPv6-PD-and-Static-IPv6-Addressing] (https://help.ui.com/hc/en-us/articles/115005868927-UniFi-UDM-USG-Configuring-DHCPv6-PD-and-Static-IPv6-Addressing)
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