If you have a PAK file from Cisco:
Router# license boot module c2900 technology-package securityk9 Router(config)# license accept end user agreement Router# reload i--- Download C2900-universalk9-mz.spa.157-3.m8.bin --
represents more than just a binary file; it is the definitive operating environment that dictates the security, performance, and reliability of an organization's digital perimeter. The Universal Image and Modular Licensing If you have a PAK file from Cisco:
The image is a robust, secure choice for anyone still utilizing the dependable Cisco 2900 ISR series. By providing a "Universal" feature set and the stability of the M8 rebuild, it ensures that your legacy hardware remains secure and functional in modern networking environments. This is a legacy release
This is a legacy release. Cisco has since announced End-of-Life (EoL) for the 2900 series and IOS 15.7(3)M8. However, many regulated environments require running the last available maintenance release of a given version for change control purposes.
Also verify RAM: