Thanks jackie_w for your support, I succeeded in "fooling" Kobo to recognize EB Garamond as Avenir.
I notice a pattern: System fonts used by Kobo are most likely popular ones created years ago, available in widely-used product and then are further modified by many designers. For example, Georgia is available in Window, Avenir Next is used in macOS. I figure out that both Georgia and Avernir Next are tweaked by designers to support Vietnamese glyphs.
The version used by Kobo seems to always be the original. This means that you can make a quick Google search to easily obtain that original font, figure out its information and then modify your chosen font in accordance with that information.