Detail workings of VPN networks and remote access

Question

I would like to have some more understanding of lower level/details how VPN (LAN to LAN connection) works. I tried to read on internet but no details are available, hence asking here.

In my office VPN server a Draytek Vigo 2910, at say office 1, I have 2 VPN settings configured in the tab:VPN and Remote Access:

1. Call direction as dial-out, Lan-to-Lan setting enabled to our second office location, where the Server IP/Hostname for VPN is mentioned as a url obtained from registering with DynDns [dynamic DNS], say xyz.dnsalias.com, This VPN link works fine and we can access the computers/intranet sites from the other LAN at office 2 from our LAN at office 1, fine.

2. Then there is second LAN to LAN setting configured with Call direction - Dial-In, with the Peer Caller ID or Peer VPN Server IP configured as the IP address of our third office location VPN server/host. This link is what does not work. What I mean is we cannot access the computers inside this remote LAN at office 3, from our local LAN. This second link was working before we moved our office recently to a different physical location locally.

My questions:

1. What settings should I be looking in the Draytek to resovle this VPN access error. I am confused about in the case of connection 1) which works, whether the Lan to Lan VPN connection is using Dynamic DNS to resolve to the VPN server IP address or is it using Static IP address of the VPN server (I hear that the VPN server in office2 have obtained static IP address for their VPN host. If it's using static IP address, what is the Dynamic DNS url xyz.dnsalias.com doing? How is it helping?

2. Second question is - I also connect from my home to my office VPN in office 2 (Office 1, where I work is still not accessible, after we have moved locations). On my home computer I have setup a VPN connection wherein I have mentioned the IP Address of the VPN server as xyz.dnsalias.com. How does it get resolved to the static IP address of the remote VPN server in office 2. Does it need to get resolved or it can just grant me access based on the hostname/url xyz.dnsalias.com?