my DDNS client, Linksys Router running DD-WRT, updates Open DNS/DNS-Omatic, but not dynu

I have long used Open DNS, updating DNS-O-Matic with my static IP. I recently added a second service to access a nextcloud server (Dynu.com) and it will not get updates from this client, and I would like to fix it.

If I use the following settings, it updates only DNS-O-matic, not dynu:

DDNS Service: Custom

DYNDNS Server: updates.opendns.com

Username: myusername

Password: mypassword

Hostname: opendns.dnsomatic.com

URL: /nic/update?

If I point my browser to https://updates.dnsomatic.com/nic/update?hostname=all.dnsomatic.com

it prompts me for a username and password then properly updates BOTH services through DNS-O-Matic. Surely there is a way to get my DDNS client to navigate the same way and update both?

Please help!