I tried hard to get this to work. I've tried avahi, and smcroute. Others have reported success, but I was never able to. One of the issues is the TTL on the multicast is set to 1, so to have it cross subnets you need to mangle it and I never got that working properly using nftables.
In the end I found rather than trying to fight with DLNA to get it to span multiple subnets, that it was easier to put an instance of minidlna on each subnet. A DLNA server doesn't have to talk to files that are local on that server. If your WiFi is running OpenWrt it should be easy. You could connect to the NAS and mount its files on the WiFi device, and then use minidlna to provide DLNA to clients on both subnets.