You really can, it's the same gauge wire in a cat6 as this. Just ensure it's weather proof if it's exposed outdoors.

I bought a normal electric cable for outside (8x1.5 m2). It works perfectly even at 50 metres. I don't think it will work with the network cable because the voltage loss will be too high. Therefore, the cable cross-section must be increased. So from 0.5 to 1.5. LG

it is simply a copper cable with approx. 8 wires. if there is no other way, I would simply buy a second cable and solder, clamp etc. the individual wires together. Depending on the tools and skills you have. Connect the box and see if it works. Otherwise, just move the control box further into the garden. My problem was that the WIFI network outside the house does not reach far. We then used a power adapter to bring the network outside and set up our own WIFI network in the garden. All the electronics are now outside in their own box. But it works very well. I control the irrigation in Spain from here in Switzerland and the garden is 2000m2 :) .