edit: if you're using the older version of LMS that doesn't use perl.exe just make sure whatever EXE it's using is allowed in the firewall.
I have checked all that and have now allowed what I believe to be relevant to LMS.
But, I just had a new thought:
I'm running an LTSC version of Windows 10.
While it is the most stable install I have ever used, and it never posed any issues, I am now thinking if it perhaps has some 'multimedia/UPnP' stuff disabled or even completely absent.
That could explain why it works for you but not for me?
And the search continues…
edit:
For what it's worth:
These two locations had UPnP/media-streaming features disabled:


So I enabled both of them.
Still no luck, but it probably won't hurt coming closer to maybe get things working.
edit:
@BoringName
Since things are working for you, could you confirm if you have these settings activated or not?
Since allowing UPnP features in the OS and routers is usually considered to be some security risk, I would rather disable it if not strictly necessary.