Can't override user agent on Orion Browser (iPhone)

Steps to reproduce:
I need to override user agent by using javascript, but it's getting error & i can't accomplish this.
I'm following this instruction:

I try to override using navigator.__defineGetter__, Object.defineProperty, Object.create but all of them still failed.
Note: It works fine on Firefox & Chrome

Expected behavior:
Can dynamic override user agent by javascript without enter Settings > User Agent > Custom

Orion, OS version; hardware type:
Orion on iOS, 1.2.7 (5) (Webkit 8613.7.1.0.6)

@Vlad Could you let me how to solve this problem ?

Vlad I know but is there any way to override this value by javascript? I would like to dynamic/ auto change the user agent by specific conditions in my application, not depend on user settings (in Orioin > Settings > User Agent > Custom).

Note: I just want to override the value of window.navigator.userAgent (some where call this statement and it will rertun the new user agent), i don't care a lot about which value server will get, just local override.

I see it's easy to do in Firefox & Chrome, but with Orion, I don't know to do

Vlad I didn't try on Safari mobile, but It works on Safafi Macbook, you can see the result in attached screenshot.

Vlad Updated. I tested on Safari iPhone and it still works, i can override the user agent