Hi Tinker,
I'm going to divide my answer to your problem in two parts below, so make sure to read both.
Part#1
There are still some things that I'm not clear on about your setup (see part #2 below to this answer), but with the information I have so far the leading suspect for your problem is the Android Oreo update has caused a compatibility problem with the HDMI cable you're using. Our App and Browser version of Cathe Live don't have any a single word of code that pertains to the output of video files through an external HDMI cable. This is controlled by the cable you use and your phones operating system.
After doing research on the effects of the Oreo update on Samsung S8 phones, and many others, it seems like problems with the USB connection are all too common. Sometimes the issues have to do with the phone not getting power from a charger while video is being played over the cable, and other times it is just a software bug that causes the video to stop playing. Two suggestions we encountered are:
1. Use a USB-C to HDMI cable that also has the ability to plug into a charger so that the phone gets power while showing video over the cable. This was not a problem before Oreo.
2. Try different USB-C to HDMI cables or converters for Oreo. Some work better than others, and some are known to have been "broken" by the Oreo update.
Here are a couple links to some discussions about USB issues after updating to Oreo...it is pretty common:
https://community.verizonwireless.com/thread/944693
https://forums.androidcentral.com/samsung-galaxy-note-8/858513-usb-c-hdmi.html
One possible solution for you is to make sure your HDMI cable is plugged in to a power source while you are trying to stream a video. From the type of cable I believe you have there should be third cable that you hook up to a power source.
Can you or have you tried streaming a video with this power cable plugged in? This seems to have helped some people that have the same issue as you. This is only a possible solution as we don't have your exact phone and cable to test.
Part #2
This part of my answer addresses some possible causes to why you can play the low quality version on the web but not the high quality, but I need some clarification from you first to really make sure my answer to you is right.
1. You say you don't have a wireless router. If this is true it means your internet speed test doesn't apply as you have to be using your cellular data 4LTE (or less). If this is the case you're getting at best 10 mbps download speed and perhaps far less. This could possibly explain why you can only play the low quality video and not the high quality video through your HDMI cable.
2. You said your download speed was 653 mbps and your upload speed was only 8 mbps. It's unusual to have an internet speed this high and also very unusual to have such a high download speed and an upload speed of only 8 mbps. I also don't see this high of a download speed being offered with your plan. Your router should also include wireless with a plan like this. It's important that we get clarification on this because we need to know if your problem could be related to your wireless router if you have one and/or your internet speed.
Can you double check this and let us know?
Hope this helps. Please get back to us with answers to these questions or with any questions you may have.