Once installed, these fonts won't change your system menus (like the Settings app), but they will appear in the font pickers of creative apps: Install and manage fonts on iPhone - Apple Support
iOS already comes with high-quality Unicode Bangla fonts. You don't always need to download something new just to type. ios bangla font download upd
Have you successfully installed a Bangla font on your iPhone? Which font do you use? Comment below! Once installed, these fonts won't change your system