Wondering if there maybe a problem with this Serviio app and newer wifi and upnp. The 1522 is based on older code and maybe less secure as uPnP had issue which were closed by later FW updates and more recent code. The 1522 hasn't been updated in a while. If you saw this issue with other products and 3rd party FW, problem maybe with this app.
Did you get to try the 880L as the main host router and use the 1522 to see if the problem tracked?
Perhaps, but Plex had the same issue. Not sure how both would fail & it be the fault of both apps. Also, when directly connected to the 895L via wifi, the DLNA shares show up. Indicating it isn't a problem with the 895L at all. I even switched to directly connecting my Xbox One via wifi to the 895L & the share shows up. The only time it doesn't is while connected to the 880L.
The issue has only appeared in products under bridge mode. I do not know the reasons why, but in DD-WRT it apparently is an issue that many people experienced & it's just been ignored since it's a fringe use (a bridge). I originally had a Rev A of the 1522 but when that went down I got a refurb from DLink that was a Rev B & that's what I'm using today. Both had no issues with seeing DLNA shares.
I honestly don't see how setting up the 880L as a router would do anything other than confirm that bridge mode is the issue. If you'd like me to do that I can, but I'm not sure how that will help with troubleshooting.