Macbook not allowing 60hz refresh rate

your computer:

2nd Display Support: Dual/Mirroring* 2nd Max. Resolution: 5120x2160*

Details:* This model can support a single external display up to 5120x2160 at 60 Hz via Thunderbolt 2.

Via HDMI video output, it can support 1080p up to 60 Hz, 3840x2160 at 30 Hz, and 4096x2160 at 24 Hz.

from:

https://everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i7-2.5-15-dual-graphics-mid-2015-retina-display-specs.html

so its built-in HDMI output can not support that display DIRECTLY at a high refresh rate.

<< change from using my HDMI input to using an adaptor to plug into my thunderbolt input. >>

NB>> You can't just choose any old adapter. You would need to use an adapter capable of HDMI 2.0 and certified cables labeled 48G to attain that resolution.

the way the display-maker works around that is by having TWO HDMI inputs, each of which can support one half of the screen, the recombine them. But your computer won't do that.

<< Is it a viable solution to buy an HDMI 2.0/48g capable adapter to get it to work at 60 hz? >>

That is your only possible solution on the computer you have -- switch up to HDMI 2.0 adapters and high-end cables.

HDMI was invented for consumer-level displays, and its use at higher resolutions than 720p requires a LOT of fiddling that resembles black magic.

The display maker also allows a different solution, not available to you, that does not compete in any way with what you want, that uses two cables. Those two cables could be using only the less-strenuous HDMI 1.4, and would not require such a high end adapter.

I am very sensitive to the idea that what gets posted here will be here for a long time, and others may find it by searching. So when I post an answer in a discussion, I try to mention the general solution, as well as your specific solution.

I am sorry if that was distracting.