You will need to change the application code or disable SNAT. REMOTE_ADDR is a CGI variable which IIS provides based on the source IP address of the TCP packet. It is not set based on an HTTP header value.
So if you're using SNAT and want to get the original client IP address, you can enable insertion of the X-Forwarded-For HTTP header on a custom HTTP profile. This dictates that LTM insert the original client IP address in an HTTP header named X-Forwarded-For. You would need to change all references in the application from REMOTE_ADDR to HTTP_X_FORWARDED_FOR.
For more info, you can check SOL4816:
SOL4816 - Using the X-Forwarded-For HTTP header to preserve the original client IP address for traffic translated by a SNAT
https://support.f5.com/kb/en-us/solutions/public/4000/800/sol4816.html (Click here