Phil, it is a very well known fact that it is hard to make HDMI work with 4k, or similar to 4K (many computer monitors have different proportions of the screen dimensions than 16:9 of 4k TVs and the number of pixels may be similar, but not exactly 4k) at a frame rate over 30 fps, if at all. Unfortunately, most “4k” cables sold on Chinazon (sorry, typo, I meant, Amazon) are Chinese junk not certified to do what they are claimed to do. On top of that, in general, high resolution video at high frame rate requires DP or USB-C (and these cables which really work are also hard to find and they cost!). If your TV supports input from DP or USB-C, try a certified DP or USB-C cable - it might help. The fact that your TV maxes out at 1080p and 30fps is a direct indication that this is all that your cable can support, regardless of what the vendor stated. When computer talks to a monitor, it automatically adjusts frame rate and resolution to what can be supported (allowed by the bandwidth of the cable).
Zwift also initially runs a test of what the graphics/monitor can support. It probably runs into a problem and shuts down because it cannot figure out what resolution can be supported. I would start with blaming it on a shitty cable.
For 60 FPS in 4k, you need a Certified HDMI 2.0 cable (or higher, HDMI 2.1). It will not be cheap (be ready for something along the lines of $50+) and it will be very hard to find on Chinazon!
Your laptop probably has a USB-C port which doubles up as a charging port, and the odds are that it could be the most capable port on your computer when it comes to graphics. I think Dell also calls it a “Thunderbolt” port, which is for practical purposes, a high level version of DP in USB-C form factor. Your mini-DP port might actually not designed to support full 4k and may be designed for a secondary monitor. It can get tricky with Dell laptops ports (don’t ask me how I know ). It is so much easier with full size GPU cards used in desktops!
Sorry, as I think while I type, I would not bet on your laptop supporting 4k through HDMI port, and as I said, it might not support it through mini-DP or DP. It might only USB-C that has its capability.
BTW, high quality USB-C cables (fully certified) are equally difficult to find. Low end just switch to lower resolution or lower frame rate.
My apologies about multiple edits (if your got several notifications)