Below are experimented techniques, which may or may not work and may not work in the future. The safest way is to create an account with a phone & SIM in the US.
What has been tried with some success in US targeting comes from using dedicated phones with VPNs (freshly reset with the app store located in the US), AWS EC2 instances in target countries, or residential proxies.
Using a local US SIM card is always optimal.
The process typically takes a handful of videos before you start seeing US locations in your analytics.
Best is to try out various techniques by yourself until you find something that works.
There is a constant stream of people talking about this issue here: https://www.blackhatworld.com/search/20087811/?q=tiktok+target+US&o=relevance.
Solution from @CastanyMiquel:
Saw a lot of interest with how to reach US🇺🇸 audience in TikTok
Full guide how I do it🔻
(change US for any other country and it’s the same process)

Prior downloading TikTok:
- Phone, no SIM
- VPN → I use nordVPN
- Change settings → region to US + put phone hour to region selected

Using Wifi / 5G is fine, no need a dedicated ip address (this might change if you’re in a high risk country for TikTok e.g. Russia, India as they monitor + those connections)
- With the VPN on now you can download TikTok
- You should see the number prefix wished
If you don’t see the US prefix → something is not working

- Create account using mail
- After joining you should see a US fyp
how? Locations, types of content. You’ll clearly see if you’re in a US fyp or in a French one.
If the language preference popup appears select region desired main language + english (so you understand something in case you want to reach Germany e.g. lol)

Warming account? Well, sometimes I do, sometimes I don’t. For me as much as waiting 3 sessions to post works fine, no need for 3-4 days. Time is money $$$
Just don’t be a bot those first 3 sessions.
How can you see you’re targeting the wanted region?
Tiktok analytics → you should see this
Put your profile on creator mode! Now under “TikTok Studio”

And a little trick for first days→ enable profile views
Even if you don’t get likes you can have data from here and stalk what profiles entered your profile → you can see if they’re American or not.

Keep VPN always on for the phone using that account.