AVR external crystal not working with UART
double check the following things:


is your baud rate calculation/register setting correct
does your crystal support the selected baud rate (is the baudrate error below the critical threshold)
is your F_CPU setting correct (calculation of baud rate register value may depend on this)
have you correctly set your fuses to use the crystal
have you mixed up RX-TX line connections (look at my comment above)