Sadly, Apple did some server changes, which mean that the ffapple method is essentially patched.
However, in the past, users could use the ffapple proxy in order to install revoked Tweaked Apps for iPhone.
Tweaked Apps iOS 14 | Install REVOKED TutuBox! Right now, it is not possible to Install Tweaked Apps for iPhone & iPad, mainly because stores such as TutuBox, AppValley, AppCake etc are revoked.