If the issue is a desire to be in control of your own router, then is there an option for a small block of routed IPv4 from Gigaclear.
Then you can use their ONT/router to handle the IP block and pass some of the IP addresses un molested to your own business grade router/firewall device.
As Gigaclear probably use remote monitoring to ensure no widespead network issues they may require their specific ONT on the end of the fibre. The answer would be to ask Gigaclear.