Error 504 DNS Lookup Fail

Hi, I'm using FG200D and FG60D, both in webproxy and sometime users get error 504 DNS Lookup fail.

in DNS setting it's use FortiDNS servers.

Also I had 3 ISP, wan1 and wan2 in WLLB mode and ISP 3 direct route to a specific devices.

Can I setup several dns server? Each ISP has it's owns DNS server (6 in total), can I use them instead of fortidns ip?

thanks.