Yes, you can install a Font (TTF) on an iPhone or iPad. But be aware that the result of installing a font on these devices might disappoint you.
The font will be working in some sections on your device, but the font cannot be applied within all the Apps you have installed. So, unfortunately, this is not something we can change for you.
The only way to get the font working in your favorite App would be if the developer of the App will install the font within the App.
To explain how this works: the Dyslexie font is a typeface like any other typefaces you have on your computer Apple or Windows today. All programs running locally can load the typeface from the font folder and you can work with it.
With an iPhone or iPad device, Apple has closed the option to add any typeface to the iPad and iPhone for years. All these years app builder has built-in their own typefaces in the apps. Now has Apple open up the last two years this feature so you can add a typeface to the system through an app called AnyFont.
The apps that do load from the font folder like on a regular computer is very limited because all app builder is used to build in their typefaces. The apps that do a load from the font folder are. Numbers, Keynote, Pages, MS Word apps
The Dyslexia Office provided by Dyslexie font can be used on your iPhone or iPad, but will only be available for you to create and type documents, not to use the font in all other Apps.