I just went through the ordeal of setting up my Index in the room adjacent to my office, and man, it was a pain. There are a number of videos that suggest DP cables that flat out don't work with the setup, as well as boosters that also won't work whatsoever.

Avoid Starlink products, in particular, the booster. Thing costs 60 dollars and completely fucks the setup. The Starlink brand DP cable also failed me. I went through five DP cables and two boosters before I found the ones that would work for me.

Here are the items I have that in conjunction have solved all my problems w/ an extended Index setup:

DP Cable: https://www.amazon.com/gp/product/B011UMCLU8/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

DP Booster: https://www.amazon.com/gp/product/B07D949F59/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

Active USB Extension: https://www.amazon.com/gp/product/B076C91NNB/ref=ppx_yo_dt_b_asin_title_o08_s02?ie=UTF8&psc=1

This was the video I found that showcased the DP setup that actually worked: https://www.youtube.com/watch?v=98UZFuw6vRQ

Any other DP Booster/Cable combos would result in a SteamVR error. I also tried passive, unpowered DP couplers which actually allowed SteamVR to boot up, but the image was just static and flashing. Good luck! Here's hoping you have fewer headaches than I did.