RF receiver becomes weakened after motor turns on
That behaviour is to be expected. Motors are very noisy and cause massive amounts of interference in radio signals.

The best thing you can do is to try and suppress that interference. For this you will need three 0.1µF (100nF) ceramic capacitors.

One capacitor is connected between the terminals of the motor to reduce noise transferred down the wires (wires make good antennae). The other two connect between each of the terminals and the metal body of the motor.

The metal body of the motor then becomes a shield to trap the noise.