Yes, ZTE is the latest one from Singtel.
No, you can not replace the ONT by yourself. And ONT life span is very long. Unless you encounter into issues it will still be okay. If you encounter issues or if you really want to change, you can get one to one replacement from Singtel. Just go to SERV+ or designated Singtel shops. It is said ALU or Ecrisson ONT may have a bit better performance than ZTE in terms of bufferbloat. But I do not feel the difference of my ZTE ONT with my older Ecrisson ONT.
https://www.singtel.com/personal/support/replacement-centres
For ONT replacement, bring the following.
ONT and its power adapter
Fibre patch cord
your NRIC/identification card1