i had been diving into this issue.
As i too want to get a new GPU (either a 4080 or 4090)
at first i was going to get the same card, but if its going to be an issue.
it needs to be resolved.
but, i was reading more on people like @Ben10dom1 had, and on the 8KX(5K Super too)
with the 3080 ti/3090
It could be that he does not have a fiber optic cable, and its a cable compatibility too. 
been reading it on here: 3090 Ti + Pimax = Broken? - #58 by Gullrum
maybe he needs a optical cable sent to him or something.
(have support send one, But we still need to figure this out regardless)
~Todd