I had the time to implement the speedtest in my oH 3.1 environment yesterday and will update the post later today.
I updated the OP to comply with the changes introduced by oH 3.
I´m not running this solution anymore and switched to the binding made by @bhomeyer based on this tutorial.
On top i´m using the remote openHAB binding to connect my raspberry Pi 4 main instance to my speedtest VM.
I spend some time to found a solution to download the speedtest result image from the url returned and store it into an item.
I´m now able to not only send a Telegram message when the Down-/Upload goes under a predefined value but also send me the result image 
I´ll share this solution in another thread.
@KidSquid could you please check your Windows part and what needs to be changed for oH 3?