Have you applied a jailbreak to your iPhone? If so, then yes, it’s reasonably straightforward to sideload banned apps, like Parler.
If not, it’s more complicated but not impossible.
Here’s a pimer from Wired on how to do it.
https://www.wired.com/story/install-apps-outside-app-store-sideload/
If you’re tech savvy, then it’s doable. If you’re not, then I wouldn’t even attempt it and instead, just stick to the browser interface for Parler, which is still accessible through Safari on iOS and MacOS.
Crap-—I have a new company Iphone and can’t do it. I’ll download the APK for my Android tablet.