No Internet works at a client where isp have fix ip and dns when I try fortidns

Can some one tell what I'm missing as at a client I want to use fortiddns but they have fix ip from isp.when I use fortidns Internet won't work on local pc . Other issue I faced with whenfixed isp ip is on vpn when I use automatic dns.internet don't work for vpn user unless ip put fixed dns 8.8.8.8 for vpn user. Please anyone guide me on this as I don't get this any issue when isp use pppoe.