Well, I don't know if I understand the question correctly, but a corresponding adapter is included with the product.
What I would be interested in is how you want to access your Synology from "outside". I use Tailscale but maybe you have a better solution?

The NR7102 already has a POE injector supplied.

Thank you very much. I will operate the LTE modem in bridge mode and direct all traffic to the Synology. I access the Synology router via Quick-Connect (Synology Cloud) and use the integrated two-fold authentication.