I have a similar problem with this router and my dns320-l dlna. For a while, he instantly finds the dlna service, but after a while, it takes a long time to find him on the network. If you make any changes to the router and save it, vlc finds the dlna almost instantaneously. The solution is to downgrade the firmware?
I'll try to do it too, but I think it's necessary to re-evaluate this router firmware failure and launch a new update.
Router specs:
DIR-826L
Hardware A1
Firmware 1.06.B01
I'm located in Brazil