Looking at the manual, the setup on the router side should be as simple as flipping a switch to enable it and setting the username/password/PSK you would like to use.
The difficult part is going to either be
A) ISP issues (such as the bridging concern Mr. Nutz brought up, or the fact that some ISPs are unfriendly towards VPNs)
or they are going to be (and this is where my money is, generally)
B) Involving the setup and environment of the remote side. The good news is that with an iPhone, there is plenty of documentation to throw at you. I have a link to the Apple doc below. In order to make this as simple as possible, you should test the VPN (at least initially) with wifi off, as I am fairly certain that no cellular provider is going to have any issues with VPNs.
https://support.apple.com/en-us/HT201550Please note:
D-Link L2TP over IPSec = iOS L2TP
D-Link Username = iOS Account
D-Link PSK = iOS Secret