Wanted to circle back to folks here just in case this might help someone else.
To recap:
I'm installing two cameras on my network.
I have an ASUS router running the ASUS software
I'm using a commercial VPN for my OUTGOING internet traffic
I needed to set up
DDNS and a VPN for my INCOMING internet traffic to secure the cameras from possible intrusion by bad actors
The combo of the commercial VPN and DDNS wasn't working
Solution:
My commercial VPN provider (Express VPN) had a setting I didn't know about that allows me to tell it the DDNS address I set up and to authenticate based on that, regardless of which external VPN server I'm using. Once I set that up, everything else fell into place and I now have functional DDNS and OpenVPN set up on my router, which I believe should protect my cameras from intrusion as I wanted (if someone sees a hole in my logic here, please do tell me).
Thank you very much to everyone who helped me. I hope that this post will help anyone else in a similar situation.