Mobile app for VPN

Good morning! I have issue about my mobile vpn for fortigate, it doesn't resolve DNS name. I checked all the settings, everything is fine, the DNS server is specified, but the mobile application does not see them.I can connect by IP address but not by domain name. here is my problem, I ask you to help.When i connect to VPN by ForticlientVPN for mobile I can't reach web resources by DNS name, but with ip adresses it's openning