Setup Unfi with extra VPN Router,

Hi guys, just want to ask some technical support. Currently my client's side is having Unifi 20mbps Dynamic IP. TM provided them Huawei echolife850a and d-link DIR615. They requested their D-link to be change to VPN Router because they want their stuffs to remote back to office to update office works.

Ok. I know D-link router is using VLAN tagging and is impossible to use other custom router to replace because mostly custom router doesn't have VLAN tagging features. If i can get normal VPN router, how do i intergrate with my existing D-link router? I got experienced in setting in Static IP by pointing ISP Gateway to custom router. That's work fine.

For dynamic IP, is it some sort of NAT or routing need to be configure? Thanks for all the advise