I cannot get safari back as my default browser even though it is checked as my default….

Ok, I understand the problem is with an iPhone. I will ask the site Hosts to relocate your question to Using iPhone - Apple Community. Until then please determine if the following information is helpful.

In Settings > DuckDuckGo > Default Browser App, Safari does not appear as a choice. Is that correct? In that case please refer to this excerpt from Change the default web browser or email app on your iPhone, iPad, or iPod touch - Apple Support:

"If you delete a web browser app, your device will set Safari as the default browser app. If you delete the Safari app, your device will set one of your other web browser apps as the default. To use the Safari app again, you'll need to reinstall it on your device."

Assuming that information is relevant, first please determine if the Safari app remains on your iPhone or not. Refer to Find your apps in App Library on iPhone - Apple Support.

• If you can find Safari drag it back to the Home Screen as it says.
• If you cannot find Safari, download it from the App Store by following this link, using DuckDuckGo or any browser, or just by clicking it in this email message: https://apps.apple.com/us/app/safari/id1146562112

Please let me know if this fixes things.

I understand your System Preferences or Settings are such that General indicates "Default web browser" is Safari. Is that correct?

Please determine if the same problems occur in "Safe Mode": How to use safe mode on your Mac - Apple Support