Your question has already been answered, as well as can be answered without knowing specifically which TV you bought, but for posterity, hopefully this will clear up the issue for Sherdoggers.
As has been mentioned, and can be observed from the HDMI version comparison chart via Wikipedia I've linked below in this post, the only time you need HDMI 2.1 is if you have a 4K TV that natively supports 4K@120Hz. Historically all you had to look for was the keyword combination "native 120Hz". Today, however, you must be careful, because many 4K TVs will advertise even "native 120Hz", but in fact, the TV can only support native 120Hz when the image is rendered at 1080p. If the TV's specifications don't include HDMI 2.1 ports you know that you're dealing with one of these, and you don't need HDMI 2.1. The below Rtings table I customized is a gut punch. There are a million units with "Native 120Hz", but of those they've tested in their lab, only two support Native 120Hz at 4K (the Samsung Q900/Q900R series and Samsung Q90/Q90R series):
https://www.rtings.com/tv/tools/table/21369
Furthermore, to iterate what has already been said, be aware that virtually no content out there runs 4K@120Hz. Netflix, Amazon, Hulu, YouTube, and all the network or cable TV channels don't. Only certain 4K Blu-Ray players do, but even then, I don't believe any of the current 4K Blu-Ray discs are actually encoded at 120Hz, today. Besides,
The Hobbit was the first major shot at 60 frames per second, and unless I missed the headline, I still haven't heard of any shot at 120Hz. So unless you're watching home video take from a phone or a digital camcorder shot at 4K@120fps, what is there to watch?
As you can see in the Rtings article below, only high-end gaming PCs and the Xbox One X or Xbox One S output this.....
but....the Xbox One S only does media playback natively at 4K, not games, and so you run into the same problem Blu-Ray players do. Meanwhile, for the Xbox One X, you run into the same problem again for games, anyway! Unless I'm mistaken not a single Xbox One game actually runs above 60fps in 4K (and frankly it wouldn't be powerful enough to keep up if they did). So unless you game on a $2K PC there's....nothing. HDMI 2.1 is still almost entirely a theoretical standard waiting on the future to catch up.
https://www.rtings.com/tv/learn/what-is-the-refresh-rate-60hz-vs-120hz
Moving on, generally, the manufacturers employ nomenclature for their refresh rate grading, and these are the ones that do/will indicate 4K@120Hz:
- LG -- TruMotion 240
- Samsung -- Motion Rate 240
- Sony -- MotionFlow XR1440
- Vizio -- Effective Refresh Rate 240
- TCL -- none*
*I pulled this from a CNET article written last summer that says that the TCL 75" 6 series unit is their only model with native 120Hz 4K)
Latest version of HDMI is 2.1. Wiki summarizes the standard succinctly:
https://en.wikipedia.org/wiki/HDMI#Version_2.1
HDMI version comparison is a nice chart just below in that Wiki (direct link here):
https://en.wikipedia.org/wiki/HDMI#Version_comparison
If you do spring for an HDMI 2.1 look for this certification label. More in depth info on the standard in the first link below:
https://www.hdmi.org/spec/hdmi2_1
https://www.hdmi.org/spec21Sub/UltraHighSpeedCable
There are slightly cheaper offerings on Amazon, but this one (for $20) was the cheapest I could find that advertises compliance from the retailer:
6.5ft 8K HDMI 2.1 Ultra High Speed 48Gbps Cable Compatible with Apple TV Roku Netflix Playstation Xbox One X Samsung Sony LG