Just changing SNAT settings to none is not going to work here. In order to work this, you should have backend server default gateway pointed to the F5. Then only it will work or else it will create asymmetric routing issue and url won't work. It seems you are having the same issue.
To achieve your requirement,
1. You can enable XFF settings to preserve the true client IP when SNAT is enabled. Below article will help you on it.
2. If you want to try with disabling SNAT settings, then make sure backend app server default gateway is pointed to F5. But it will cause your internet traffic also sending to F5 first. So you need to take that into consideration.