I didn't know that my Synology NAS has a dedicated section for Dynamic DNS services. Select the DDNS tab from External Access in the Control Panel.
I usually use a Mikrotik router which handles DDNS for me. During outages I switch to the ISP-supplied router as it has 4G backup so lose DDNS access. The NAS is router agnostic, and always on when I need it, and seems to be the perfect device to handle DDNS.
I've configured it to use Duck DNS along with Synology's own offering. Now, whether I'm on my main connection or backup, DNS updates just work.