Depending on how many other users you have at home, how many devices in total and what kind of reliability you need, you could go with the ISP-provided modem+router (cheap, low/moderate reliability, terrible QoS and even more terrible wifi due to placement issues) or upgrade.
If you have a budget of over 15K for networking equipment consider using the ISP modem (GPON) just as a dumb pipe to your own router. Then get a prosumer router such as the
USG and then get one or two
access points that are connected to the USG.
This will give you rock-solid reliability for tons of simultaneous devices over wifi, great QoS when your're trying to have a video call while the family is watching netflix, and great security.
This also has the benefit of isolating your internal network configuration from your ISP, so you can change ISPs (or have two of them connected to the USG for reliability) with no changes to your internal network.
You must be savvy about networking to make use of the advanced features of the unifi gear though.