What to use to cut large roots
If you are comfortable using power tools a Sawz-all also called a reciprocating saw will do the job. You will want a coarse blade with less than 14 teeth per inch. They are  available in battery powered models if you don't have an outlet nearby. If you have never used anything like it before ask the salesperson if they can show you how to use it safely. Ask if they have a piece of scrap wood so you can make a cut to see if you are comfortable using it.