Best option is to look at using better antennas, or better orientation of the antennas.

Another thing is to verify that you have very smooth power for the receiver and transmitter.

The next step is to try to make the transfer channel more immune to noise, i.e. looking at using lower baudrate. Looking at filtering the serial data from the receiver module. Looking at transmitting the data with redundant error-correction codes using either multi-dimensional parity bits or ECC.

Have you looked with an oscilloscope at the data output from the receiver? The look of the signal can give a lot of ideas how the signal may be filtered to better extract the serial data.