Hello. Relatively recently, a problem appeared: the router does not issue identifiers to clients. This has never happened before and everything worked fine, but now when turning on computers there is often an “unidentified network” problem. This problem does not occur all the time, it can happen on the same computers or on different ones, but not on all. I tried to reset the router to factory settings, but it didn't help. The only thing that helps is removing the client from the list of leases and reconnecting him, and so on every time. What could be the problem?

Facing same problem, still not solve

Client ID is something DHCP clients supply to server (and not the other way around). When DHCP server decides on which lease to offer, it first checks "Client ID" ... and only if that value was not provided by client, it falls back to using client MAC address (as identifier). Indeed most DHCP clients construct "Client ID" using own MAC address, but it doesn't have to be. BTW, "active host name" is also provided by DHCP client in ROS case (ROS DHCP server by default doesn't send Option 12 (Hostname) to clients).

Regarding "unidentified network": it's windows internal stuff and most probably it's based on several information pieces (including but not limited to IP and MAC addresses of default gateway). Based on this information, windows asks user which kind of network it is (home / work / internet) and adjusts windows firewall settings accordingly. Next time windows encounters same set of network information pieces, it declares network as identified and applies same firewall settings as they were first set.
Now, if many windows clients complain about unidentified network and you know that same windows machine was connected to same network previously and you know that none of information about LAN (used by windows to identify network) is changed, then you may have some problem in your network ... e.g. rogue DHCP server.

Just a comment on screenshot, posted by @OP: I've no idea how DHCP server was able to send out a dynamic lease without recording client MAC address and ClientID (row, pointed at by red arrow).

The "active address" in that row seems to be assigned to MAC address 00:00:00:00:00:00, so maybe the base issue is not the missing client id, but the (empty) MAC.

The "active address" in that row seems to be assigned to MAC address 00:00:00:00:00:00, so maybe the base issue is not the missing client id, but the (empty) MAC.

I can't imagine how could this happen. DHCP server does receive "DHCP discover" and "DHCP request" packets from client and for client to be able to communicate over ethernet, it has to have valid MAC address (the whole thing is different from ARP table where there can be entry with legitimate IP address and "all zero" MAC if ARP whohas doesn't yield a result). The only way for DHCP to succeed without valid MAC would be if client was behind an L3 tunnel ... but I don't know if it's possible to bind DHCP server to such interface.