I upgraded to the firmware provided by
@EMPIRETECANDY here (click the little up arrow next to his name to go to post)
and now I am able to successfully subscribe to the IVS notifications of the NVR device on my Android (gDMSS Plus version 3.49.003). I tested this on my LAN and also connected to my VPN and it works on both. Maybe even better than before.
Dahua has had a few issues this year since I got all my equipment (IVS skipping frames and the IVS pushes breaking) but they seem to be relatively quick at getting fixes out. I know
@EMPIRETECANDY has sent some of our concerns to Dahua engineers, so thank you for that as I'm sure it helped move things along.