If you do not have access to DHCP configuration on the ONT then your only option is to insert your own router in between the ONT and the rest of your network.

• Are you provided any administrative functions for the ONT?

In most cases, these devices offer a "bridge mode" that let's you put a router behind it, so it works more like a modem (disabling its built in Wi-Fi, DHCP, etc). That's likely what you want. It's possible you will need to contact the ISP to put it in bridge mode if they are particularly restrictive about device control.

Yeh, you get a router you actually manage and don’t use your ISPs shit.