For a discussion regarding how to sign the concept of a "mortgage," see: MORTGAGE
In general you can just do the sign for "loan" or you can fingerspell "mortgage." is usually just signed as "LOAN."
See: LOAN
To convey the concept of a "15-year fixed rate mortgage" you can sign:"15 YEAR LEVEL-FLAT-extending-onward LOAN"
The concept that I'm calling "LEVEL-FLAT-extending-onward" is a sign that is commonly used to mean that there is a long, flat surface extending in front of you, such as a road cutting through a desert. This concept applies well to represent the idea of an interest rate that likewise is "flat" (doesn't go up and down) and extends forward (into time).
Notes: