Hey Andrea,
I hat the Same issue when starting with NS.
Ist depends how you reach from WAN your NS.
Lets say you have set a DNS record named remote.example.com and you enter your office and be from That on inside you LAN, your phone cant find the Server except you have set the Same record in your local DNS.
That solved my Problem back then.
Two same DNS records in LAN and WAN also solves the certificate issue regarding IPhone.
Regards
Thomas