Standard virtual server:
1.URL is not accessible
DNS—would be first troubleshooting
Nslookup to URL it is gives you timeout
That would be first troubleshooting step
2.TCP DUMP
3.TCPDUMP on linux
4.TCPDUMP you will see whether you are able to see session from client
Client to F5 and one is F5 to server
5.you need to ask them can they directly access the backend server