Certificate Warning captive portal with FortiAp

Hello, I configure a Fortiap to access internet by my fortigate. A certificate warning appears when internet browser tries to connect to captive portal at the following address [link]https://172.16.40.1:1003/fgtauth[/link] where ip address is fortiap access point. I follow the link http://cookbook.fortinet....l-wifi-access-control/ to configure fortiap and fortigate 600c (rules, network, user access group,...). I don't know if I need a certificate based on ip address. If I have 10 fortiap I will need 10 certificates, it is not good. I saw that it should be necessary to configure fqdn address for captive portal but I don't know how. When I've configured wifi SSID I tried to set DNS parameter to "same as DNS system", "same as IP interface" or "specify" and the only which is working is to specify DNS address 8.8.8.8 (can resolve DNS requests). I don't know if it can be a part of solution. Can you help me to solve my problem. Thanks.