cepix wrote

I connected 20 computers to the switch. Nothing else (nor router, internet or any other network device). For some reason during this installation, the range of IPs assigned changed from 169.X to 192.X

Uhm, the 169.254.0.0/16 IP range is allocated to the "link local block" according to RFC3927. Your systems get an address from this range by auto-configuration if there is no DHCP server running in your network. The 192.168.0.0/16 IP range is the IP address space for private networks assigned by IANA. This range is often is used by DHCP servers to dynamically assign an address.

The TL-SG1048 is an unmanaged (dumb) switch. It neither does auto-configuration, nor does it have a DHCP server. Auto-configuration is done by your OS and DHCP is done by your DHCP server.

Question 1: Why IP range change suddenly?

See above.

Question 2: Why IPs got stucked in a wrong IP range?

It's not a wrong IP range, it's a private range defined in your DHCP server. Still has nothing to do with the switch.

cepix wrote

So If i connect a home router (linksys WRT54G) It can serve as DHCP server and all assigments will be done using this device?

Yes. But make sure to disable the other DHCP service in the same subnet and use a non-conflicting (free) static IP for the WRT54.