This is only one of many threads in many locations about networking problems with the 24H2 update. I am posting because I haven't seen a report elsewhere of the fix that worked for me.
It seems to be a well-kept secret that Microsoft has call-back telephone support, staffed by reasonably competent, albeit low-level, techs. It is free if you have a MIcrosoft 365 license. You can reach it via support.microsoft.com by rejecting automatic fixes to problems. In my case, I asked for calls twice (the initially suggested action, which didn't help, took a long time, so we had agreed that I would 'call back' with the system in the state that demonstrated the problem). The first request was 'returned' within a few minutes, the second within an hour or so.
After I installed the 24H2 update on my primary desktop system, my wife's Surface could no longer see it on the local network, nor connect to the mail server that is hosted on it. My desktop could no longer see the NAS (or any other computers) on the network, but both systems could still connect to websites on other local devices, and to the general internet, with no problems or issues. It rapidly became apparent that my desktop was failing correctly to identify the local network to which it was attached, and was registering it as an 'Unidentified' Public network. There were no available settings options through which this could be changed. My desktop is normally hard wired to my router - almost all my other devices are connected to it via a WiFi Access Point. I had found that if I disabled the Ethernet connection and used WiFi to connect the desktop to the router, the network was identified correctly (as a named Private network) and, after re-enabling file and printer sharing, everything started working normally again. To cut quite a long story short, the second technician confirmed my observations and tried quite a lot of potential 'fixes' by remote control before looking at my Ethernet port drivers. I was using current Intel drivers for my (Intel chipset) system, and he switched to the latest Microsoft Windows 11 drivers. This solved all the problems! On reconnecting to the network, it was recognized as my named Private Network, and (after again enabling file and printer sharing), all the connection problems were resolved. The 'fix' appears to be stable through a number of reboots.
I assume that 24H2 makes some assumptions about the capabilities of the networking drivers that are true for Microsoft's drivers but not necessarily so for those produced by other manufacturers (probably before the release of 24H2). This may explain at least some of the major networking issues that have surfaced in a release that was (presumably) thoroughly tested - Microsoft's test systems probably use their own networking drivers!