For your information, every standard broadband installation in Malaysia comes with a modem and a router.
To put things short, the modem connects to the internet. So, you can’t the change the Unifi Modem (most of the time even Maxis is using an Unifi Modem)
The router establishes local area network either through LAN or WiFi. And the router that Unifi/Maxis give to the customer for free is an entry-level router. This makes sense for them to save cost. And high-end users can replace the existing router on their own for better internet experience.