Hi, Chin,
Some accounts still have Times New Roman and some do not. Unfortunately, there isn't any way to add it. If it's not showing up, even for documents that previously used it, then it appears that it's gone from your account.
Please send a request for a return of this font directly to the Docs team by opening a Docs document and going to Help > Help Docs improve. The team is unable to respond to individual users, but they do review all feedback sent to them and use it to improve the app over time.
In the meantime, I would suggest using the "Times" font. Here is a screen shot of two paragraphs: the first one is in Times New Roman and the second one is in Times.

It would take a seasoned font expert to discern any differences.
~Jo
Google Product Expert Volunteer (not a Google employee)