Here are few suggestion that may help in solving the issue.
- Start by reseating the CMOS battery, remove it for couple of seconds then put it back in then see if BIOS would work normally.
- If these doesn't work do also try to install the latest BIOS update.
- http/www.dell.com/support/home/us/en/04/product-support/product/inspiron-15r-5521/drivers/advanced
- Just select the correct OS and download the latest BIOS update and see if this will solve the issue.