lan network with internet

we have a unconfig switch 16 port. 8 client. 1 server. internet modem router. this small LAN network take IP address automatically and work properly. their IP range from 192.168.1.1 to 192.168.1.255. when we turn modem router on, network failed and IP range for server and client changed to 169.254….. and when we turn modem router off after server restart they work properly. now we gave them static ip and now all things is ok. why when we give them automatically ip, network failed and client and server ip range, changed?