The two files you need can be found in below links:
AS_5500_RTOS-7.0.116.0 = load for 'RTOS'
AS_5500_cavium_main-7.0.116.0 = load for ‘FP'
The provided RTOS images are compatible with 2504, 5508, WiSM2.
I also list the procedure below for your reference:
1. Copy the both RTOS files to a reachable TFTP server.
2. Reset your controller.
3. Keep pressing ESC until you're at the Bootloader Menu.
4. From the Bootloader menu, choose 6 (Manually update images).
Proof of life:
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Boot Loader Menu
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1. Run primary image (Image not found) - Active
2. Run backup image (Image not found)
3. Change active boot image
4. Clear configuration
5. Format FLASH Drive
6. Manually update images
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Enter selection: 6
Launching...
Launching images...
init started: BusyBox v1.6.0 (2009-02-03 04:56:32 EST) multi-call binary
Mount failed for selinuxfs on /selinux: No such file or directory
starting pid 655, tty '': '/etc/init.d/rcS'
Use DHCP for ip configuration (Y/n)? n
5. Enter your controller Management IP address, Subnet Mask, Gateway, and
TFTP server IP address.
Enter switch IP address: 192.168.1.100
Enter switch netmask: 255.255.255.0
Enter switch gateway: 192.168.1.1
Enter TFTP server IP address: 192.168.5.157
You have entered:
IP Address : 192.168.1.100
Netmask : 255.255.255.0
Gateway : 192.168.1.1
Server Ip Addr : 192.168.5.157
Is this correct(y/N)? y
!!! WARNING updating using .aes or unapproved files will disable this unit
!!!
Do you want to update RTOS (y/N)? y
Do you want to update Primary Or Secondary Image (P/s)?
6. When it prompts you to enter a filename for RTOS update, enter the name
"AS_5500_RTOS-7.0.116.0"
Enter filename for RTOS update: AS_5500_RTOS-7.0.116.0 RTOS update Done
7. When it prompts you to enter a filename for RTOS update, enter the name
"AS_5500_cavium_main-7.0.116.0"
Enter filename for FP update: AS_5500_cavium_main-7.0.116.0
FP update Done
Do you want to update an AP image (y/N)?
AP Images Not Updated
Done. Restarting...
Restarting system.
Then you're done, all you need to do is wait for the upgrade to be finished.
Please feel free to let me know if you face any issues in the upgrade.