Good day!
I can setup VPN server on your DrayTek and VPN client on your iPhone qualitatively, because I am certified network engineer MTCTCE/MTCNA having over 10 years of experience in configuring, maintaining and troubleshooting of enterprise and SOHO network devices (including DrayTek), VPN's and firewalls.
What type of VPN You need (PPTP, IPSec or L2TP with IPSec Policy)?
What DrayTek model You have?