LG UltraFine & replacement Belkin USB-C cable gives blurry text
Looking at the specs of the the LG 5K Display and the accessories (cable) it comes with, you can't just use any plain USB-3 cable.

You need a Thunderbolt 3 Active Cable.  Thunderbolt 1 and 2, natively, were active meaning power was required to operate properly.  Normally, Thunderbolt 3 is a passive, meaning power isn't present. Any quality USB-3 cable will carry Thunderbolt 3 (and vice versa).
However, to drive this particular monitor, it appears LG requires an active cable.