Hello, friend!
For enterprise network products:
Log in to the ONT WebUI as an administrator. In the navigation tree on the left, choose LAN > DHCP Server Configuration.Then, perform related configurations.

However, the HG8546M is a carrier series product. DHCP is delivered by the ISP server. According to your description, the configuration of the HG8546M is preset and cannot be modified later. Therefore, we cannot provide effective help for you.
Thanks!