You’ve probably done it already, but a quick howto for anyone else (on Firefox Mobile):

• Three dot menu -> Settings -> Search -> Default search engine
• Scroll to the bottom, press + Add search engine, put what you like in the Name box
• in the search string URL box put this: https://noai.duckduckgo.com/?q=%25s
• Press Save, then select your newly named option for normal and private tabs as you like.