Hello, TruongLuan
Welcome to Microsoft Community.
Unfortunately, the Android version of Edge is not like the PC version, and doesn't allow to add another search engine. If you prefer DuckDuckGo, you can manually enter duckduckgo.com in the address bar, go to the website, and then bookmark the page or add it to your browser homepage.
If you would like the Android version of Edge to allow adding default search engines in the future, you can provide feedback to the developers. Open the Android version of Edge, then select Edge settings > Help and feedback. If the Help and feedback option is not displayed, select all menus (you may need to scroll the menu section), then select Help and feedback.
As for the characters appearing as question marks, are you using a custom font or the default font on your device? Some custom fonts may have issues displaying certain characters, but the default font usually does not. If you are using a custom font, please try switching to the default font and see if that helps.
Yuhao Li
Microsoft Community Technical Support