If possible take a couple of photographs of the current setup.
Post the photographs here via imgur (
www.imgur.com).
Be sure to avoid revealing any personally identifying information.
There are a number of possible means to support the cable. However there are likely some trade-offs involved.
For example: if "too tight", you may limit or otherwise restrict the movement of the PC to where a bump would yank on the cable.
And you want the support to be easily undone for times of movement: cleaning, maintenance, changes.
Plus, if the support is visible then appearance is another consideration.
Depending on the rack and the overall setup, a simple velcro tie wrap may work very well. Avoid zip-ties, twisty wires. Avoid anything requiring tools for other than perhaps securing in an initial anchor point.
Google "computer cable management" and search "images". Very likely that you will find an idea that works for your requirements.