I know this is an old thread but I am having the same problem here on a RasPi 1 with LibreElec connected by cable to an FritzBox 6490. Strange is, that with my Win10 Laptop and an DLNA app I can easily connect and stream to Kodi but not with my Sony mobile phone nor with my Samsung Tablet.
Is there any fix for this now?
Last thing I wanted to try is to switch back to OSMC though I had less stability why I changed to libreelec.
I suggest you seek support from LibreElec then… You’re asking the wrong ppl here…
nvm. why should we help each other because we just have the same problem? Oh, you´re driving a Mercedes? Sorry dude, but wheel changing is totally different to my Porsche… -.-
BTW:
I found out that this problem in my case is cause by an additional router. Maybe it´s a hint even for OSMC users 
Sure, did you ever try seeking OSMC support on the LibreElec forum?
Anyway, enjoy your issue resolution.
Hey guys,
I came across the same problem, Kodi UPNP/DLNA server was not found on local network.
And it looks like I found the solution, permanent one. At least it works for me for a few days, so I would like to share.
Before I started up OSMC/Kodi on brand new Rpi3B+, I already had DLNA server (miniDLNA) on another RPi. So I should see two DLNA servers on my network (I skip all the steps to turn on UPNP/DLNA on Kodi). This wasn’t the case many times. While I always saw my old DLNA server, I saw Kodi DLNA from time to time. After few days of monitoring the situation I noticed a difference, thank to DLNA Browser application on Win10. My old DLNA server was running on port 8200, while Kodi DLNA was running on another port. I say another, because my server was running on port 1403, while my coleague’s Kodi (with the same problem as mine) is running on port 1334. Looks like Kodi chooses random port?
Anyway, based on this finding, I changed the port in upnpserver.xml to 8200 and restarted Rpi with Kodi. From that moment, I can see Kodi UPNP server on my network all the time, from any browser (Android BubbleUpnp, VLC player, Windows 10 DLNA Browser), even on bridged OpenVPN network.
Hope this will help someone.
Wow, this actually worked 
The upnpserver.xml is located here: /home/osmc/.kodi/userdata