AnyFont can open .otf and .ttf (even if they are in a .zip) directly from GDrive (Google Drive) using the AnyFont app on iPadOS. It's a bit convoluted and slow and manual .... but it works (should also work on iPhone).
Equipment used: iPad Pro 11" (2018)
Software in video:
Google Drive: https://apps.apple.com/us/app/google-...
AnyFont: https://apps.apple.com/us/app/anyfont...