Connection minimserver - UPnP control point unstable - issue NAS/Minimserver?

[Cebolla]

If none of what Simon suggests works, then you're most likely stuck with having to switch the wired/wireless crossing IGMP snooping off.

You may able to use the Thomson/Technicolor router's command line interface (CLI) to switch it off, via telnet. I actually did this myself years ago on a Technicolor TG582n (provided by the old O2 Broadband service), plus the webpage where I originally got the info from is still there - hopefully the same commands work on your TG789bvn :
https://lehollandaisvolant.net/tout/_misc/telnet/

Make sure you do as it suggests at the beginning and backup your router's config, before making any configuration changes via the router's CLI. The relevant section is "DLNA Server (Media Sharing) Not Discovered Across LAN / WLAN" (about 3/4 of the way down), using the eth bridge igmpsnooping config command.

For a telnet client application, you could try the Windows built-in one (needs to be enabled) or just install the excellent PuTTY SSH and telnet client:
https://www.putty.org/

Sorry I couldn't get back in time to tell you not to bother with port forwarding!