People are talking all over the place on this.
FIRST: TRY WHAT YOU HAVE!!!!!!
People believe that because the cable they already have isn't listed to support something that it must not support it. But, the reality is that many times cables that are in place already support resolutions and data rates which are beyond the specification that existed at the time which the cable was manufactured. This isn't always the case, but is most often the case.
SECOND: HDMI 2.1 cables basically don't exist. HDMI 2.1 has a 48Gb/s speed rating. It has some other features which have been implemented which are supported by HDMI 2.0 cables. But, the biggest difference, when it comes to cables, is that the speed almost tripled. Since HDMI 2.0 asks for cables which support 18Gb/s, that's the current highest level specification which can be supported across a long distance until new chips come to the market which can support the higher data rates. So, you won't have much luck in the 48Gb/s department on cables right now.
If you find your existing cable doesn't actually work, then you can look for a HDMI over Ethernet extender which supports 4K/60 signals at 18Gb/s. They do exist, and if running a new cable means opening walls, then an extender is a far cheaper option to repairing drywall and repainting.
18Gb/s HDMI can be tricky, and I'm not sure of which type of Ethernet cable that you have - cat5e or cat6? Also, the distance you seem to list as just about 25 feet or so from end to end. Which is VERY short in terms of network cable length.
I would expect that a HDMI extender would work just fine as long as it can support 18Gb/s HDMI.
This is the cheaper model:
https://www.monoprice.com/product?p_id=21792#QuestionsandAnswers
This is the more expensive model:
https://www.monoprice.com/product?p_id=21668
Neither is cheap.
If you have a crawl space and you don't need to do drywall work, and you are just complaining because it is a pain to do, but not actually going to cost a ton of money, then just replace the cable. Find out the right length, then get a new active HDMI 2.0, 18Gb/s rated cable. Test it across your floor for a week or two before you install it to ensure it works.
Understand that in 5 years or so, that if you upgrade, you will need to replace the cable again. That's the reality of home A/V.
