This is Cat 5 but it's not shield.
That's not possible, Cat 5 cable is always shielded. Do you mismatch Cat 5 by twisted-pair?
A Cat 5 cable is not the best solution to transport a non-differential 5V signal over almost 20m. In an electrically not so noisy environment I had better results with rather thick loudspeaker cables, but I had only about 8-10m.
I agree with Nick_Pyner to use another MCU near the sixth sensor. Connecting it wireless is one possibility, you have also wired options (Ethernet, RS-485, etc.).