Official and community-hosted sources for the 2015 Dual Core firmware include:
Supports almost 100% of NDS games and runs on DS, DS Lite, DSi (up to v1.4.5), and 3DS (including XL/2DS). r4 sdhc 2015 dual core firmware
Today, the R4 SDHC 2015 Dual Core serves as a lesson in consumer electronics history. It represents the "Wild West" era of flashcarts—a time when branding meant nothing, firmware versions changed weekly, and the difference between a working console and a bricked paperweight was often just a few lines of code Official and community-hosted sources for the 2015 Dual
This means your firmware does not match your hardware. You likely downloaded firmware for a different R4 model. You may need to try a few different variations (like the "Revolution for DS" firmware vs the "R4i" firmware) until one sticks. You likely downloaded firmware for a different R4 model