Hello Josspa,
Welcome to Microsoft Community.
I have read everything you have posted here in full, and your question is essentially a Hisense Smart TV usage issue, in fact, it is the most correct to consult the Hisense Smart TV after-sales support.
But see if we can give you some advice.
In short, you want to connect your Hisense Smart TV to your computer as an external screen, either via cable connection or directly via wireless projection. However, in your description, what you are trying to do is project the screen onto the TV through the APP, which is not the same as viewing your TV directly as a display.
So, the first thing you can try is to connect your TV to your computer using an HDMI or DP cable, and then look at your computer's multi-monitor Settings (Start menu -- Settings -- System -- Multi-monitor -- check other monitors), and if you can see the device directly, you can theoretically use it directly as a monitor. Perhaps you need to switch the signal source in your TV.
However, if you can't do this directly with a data cable, you may need to connect your computer to your monitor via "wireless projection" (provided that your TV supports this feature). I found the appropriate support link for you:
Screen mirroring and projecting to your PC
If so, it means that you can use this feature to connect your computer and TV, but if you still can't see it, I think you should check with the after-sale support of the Hisense Smart TV to confirm whether your TV supports contact your computer, if yes, maybe they can give you more advice.
Best regards,
Mitchell - | Microsoft community support expert from MSFT