The people using iOS devices are required to opt-in for NSFW access in their desktop settings to get access to age-restricted servers.
The steps to do that are:
1. Log in with your Discord account and navigate to user settings
2. Click on Privacy & Saftey
3. You'll find a toggle button saying, "Allow access to age-restricted content of iOS."
4. Turn the toggle button "ON."
And that's it. Now you'll get access to join and view channels in NSFW Discord servers.